Maybe it's me, but I found trying to practice two religious traditions at once an impossibility. It wasn't because I couldn't make sense of how the two went together; it was because I found that practicing what each on its own terms had to offer was the work of a lifetime. And even if I could tell myself that I was doing one thing in doing both, I knew that I stood quite alone in that estimation. In my heart of hearts, I knew that playing both sides of the street was an ego game that served nothing so much as satisfying my own sense of self.
The arguments in favor of practicing two traditions at once are as compelling to me as the arguments in favor of having multiple spouses. Yes, it's quite interesting. Yes, it brings to light things you might otherwise miss. Yes, it opens you up to being more than you could be with just one.
But none of that gets to the point of the exercise, now, does it?
(It may well be the case that one finds that one is in the process of moving out of one tradition and into another, and that one is finding one's feet for a time. One isn't ready to commit wholeheartedly in the new direction just yet; one isn't ready to untie the old moorings just yet. I get that. But at most, it is a transitional phase that sooner or later leads to one settling back into the tradition of origin or else moving on to the new one.)
Which brings me to those who would serve as spiritual leaders in more than one tradition.
I won't say that the mass celebrated by a sanctioned Zen teacher/Roman Catholic priest isn't valid or that he can't really pass students on koans. Of course the jobs can get done, in much the same way that one can be a decent cake baker and and a decent cellist. There no logical contradiction involved at all.
I won't say that the sanctioned Zen teacher/post-denominational minister can't be both a skillful expositor of the Buddhadharma on one day and an inspiring preacher on another. I know from teaching diverse courses in a curriculum how easy it is to move from Descartes in the morning to Democritus in the afternoon without missing a beat and without the students ever suspecting anything about the other course.
I will say that what I find lacking in both of those instances is singleness of heart, a truly rare commodity in a society of multitaskers. Singleness of heart isn't a virtue in someone who can't see past the end of his or her nose; it's a virtue in one who, being well aware of the vast array of highways and byways and the many treasures that lie along them, nevertheless humbly submits to the discipline of one alone and, forsaking all others (to borrow from the marriage analogy again), commits to depth rather than breadth of coverage.
Maybe it's me, but I don't know that I could fully trust or respect anyone other than the single-hearted to guide me -- or even just to walk with me -- on the path.