Greetings of Peace. Although your question is one that would be most appropriately directed toward a spiritual guide, the prevailing attitude here is that we may benefit from each other's experiences and considerations in a spirit of charity and humility without such perspectives holding a pretense of absolute authority or finality, such being the province proper to the shaykh, spiritual father, or guru alone.
The main consideration that you have expressed concerns the phenomenon of interdenominational participation. In Islam this phenomenon presents less of an obstacle due to the orthodox acceptance of different codifications of Islamic jurisprudence and theology which determine the corporate body of Islam. Although a significant tension, both doctrinally and politically, exists between Sunni and Shi'ite Islam, even this is oftentimes assuaged within the context of congregational worship. For instance, Sunnis may sometimes pray alongside Shi'ites at a Shi'ite mosque (as was once frequently my custom) and Shi'ites, who in America are a comparative minority, may sometimes find only a Sunni mosque in the vicinity with which to fulfill this obligation in which case dissimulation may have a role to play. In either instance, the Muslim in question rests assured that his activity is unquestionably legitimate, the diversity of the ummah having received the sanction of the Quranic revelation.
As an older tradition that has accumulated a more pronounced diversity within its denominations as well as more openings toward modernism, Christianity seems to require a more nuanced consideration. In Catholicism, for instance, salvation occurs with and through the Church and emphasis is placed upon the rich sacramental life of Catholic Christian rites. In a typical modern independent Evangelical Church salvation occurs through faith and emphasis is placed, as the epithet implies, upon evangelizing or communicating the words of the Bible and deriving concrete moral lessons from them.
In the example of Evangelicalism given above, and in my experience with adherents of local megachurches, services are primarily a social affair virtually void of any sacramental participation with the exception of a simple immersive baptism and dedicated (rather than consecrated) communion, all emphasis being placed upon scriptural communication and fellowship. In light of this it seems plausible that you may in good conscience actively devote yourself to the sacramental life of the Catholic Church to fulfill your spiritual needs while engaging in the fellowship of the Evangelical service to support your wife and her social needs. The contrast is such that it would not involve the same tensions as simultaneously participating in the sacraments of the Catholic and Orthodox Churches for example.
In any case, interdenominational participation does not arouse the same degree of concern as does interreligious participation as the latter involves moving within dimensions circumscribed and permeated by different revelations.
In our discourse, religions are often referred to as forms, which at least in my mind refers most especially to the manner in which all of the elements of a particular tradition (see the preliminary definition on the homepage) leave their distinctive imprint upon and "shape" the contours of the soul. One of the effects of interreligious participation is a confusion, not necessarily of the mind which is capable of compartmentalization, but rather of the psychic substance which is more fluid and shifting in nature. It is specifically the impress of this shape that allows the soul to receive the spiritual influences descending from heaven to which it is uniquely suited and from which it initially emerges. In my experience, the oscillation of the soul back and forth between different religious forms prevents a single tradition from taking root and crystalizing itself such that one's engagement is superficial at best and deleterious at worst.
An excellent parallel can be drawn between the religious form impressed upon the soul and the various distinctive canons of sacred art and architecture. Inasmuch as each person's soul is a temple fashioned according to the proportions and symbolism uniquely suited to its consecrated function whether that be Hindu, Christian, Muslim or some other form of worship, so does the importation of elements selected haphazardly from foreign canons disrupt the equilibrium of the total structure in which each element contributes to the harmony of the whole. The example that you have given of being a Christian, while occasionally indulging in salat is, according to this analogy, akin to periodically trying to introduce a mihrab and courtyard into a cathedral.
Ultimately a tradition, in order to be effective, requires our total and exclusive participation and engagement at a formal level. Although our perspective involves the acknowledgement of the unity of religions, it is a transcendent unity that is envisaged and not a formal one. Possession of this perspective is tantamount to having a window in the heart whereby we may gaze out upon other worlds of beauty and sanctity. It does not necessarily involve moving freely about them as a perpetual tourist who follows his wanderlust throughout different lands without setting up roots in any given soil.
There are certain notable exceptions that are worthy of brief consideration including Sri Ramakrishna, Huston Smith, and Frithjof Schuon - I am not familiar with the teachings or circumstances of Bede Griffiths. Each of these people possessed a unique dispensation and individual vocation which made some form of interreligious participation possible. Ramakrishna was a master of nirvikalpa samadhi and under the promptings of Kali was directed to experience the realization of the fullness of possibilities of Brahman by briefly practicing other religions. Huston Smith, through his legacy, continues to teach religion to hundreds of thousands of people, and in this capacity received permission from his spiritual master to gain firsthand knowledge of multiple religious forms. Finally, Schuon was perhaps the greatest reviver of esoterism in modern times. In his capacity of spiritual master, he took his starting point in Islam, but his spiritual influence and guidance in the domain of esoterism providentially extended into other religious universes as well.
As previously stated, these individuals represent exceptions to the prevailing perspective, that of formal exclusivity. It is ultimately for you to determine with the appropriate guidance whether you do in fact possess such a vocation or whether you are simple conducting improvisations in this field.
There is one final consideration to be made here and it is that if one is not to engage in abject syncretism, how does one manifest ones love of other religious traditions and become enriched thereby. It is first important to realize that this perspective endows one with a heart that is more open to the diversity of the manifestations of the sacred in all of its forms, which may be viewed as a great compensation arising from God's mercy in an age that is more than ever permeated by secularism and profanity. Furthermore such a heart is not only more open to God but also filled with a more profound love and appreciation of the neighbor and one is bestowed with a greater capacity for kinship and mutual understanding with the followers of other traditions. It is this quality that makes it possible to have communities such as ours where like minded people can correspond in a state of mutual respect and share with each other freely and charitably without in any way compromising their integrity or fidelity to their respective traditions.
Finally, encounters with other traditions whether through sacred art and music, doctrine and symbolism , or the observation of sacred rites make us sensitive to and appreciative of the contours of our own. Also those aspects of esoterism that are more prominent in others can inspire us to fervently investigate and aspire to the fullness of the the possibilities inherent in the interior dimensions of ours. There is a well known adage that to follow one religion fully is to follow all religions or rather religion as such and it is precisely encounters as we have considered here that help us to understand and appreciate the veracity of this realization.
I hope that these thoughts may be of assistance to you in determining a suitable course of action for yourself.
Greetings of Peace. This blog contains selections from my correspondence and other sources on a variety of subjects related to religion, philosophy, and spirituality. I hope that they may be of benefit to the interested reader. Concerning the title of the blog, read this entry.