Thanks for all of the insightful responses. It's interesting to hear a cross-section of views, and consider a few things that hadn't occurred to me.
This had never occurred to me, given the context of my relationships, but I can see how each could be a question someone else might ask. In my case, when I say "close friends," I truly mean that. As in, we've been through thick and thin together, we've seen each other's best and worst, and I would say that we trust one another implicitly. One reason that these guys are my best friends is also that they are not members of the misogynistic, homophobic, insensitive crowd. If they were, I wouldn't be as close with them as I am. Those sorts of people I keep at arms length, and regard rather warily. I also would have no reason to doubt their sincerity. I've never known them to be dishonest or deceitful, and certainly not the type of guys to play tricks like that.
Of course, I realize that the mileage of others may vary. But that is what I had in mind when I used the term "closest friends."
First of all, Rob, it makes me very happy to hear that you're that supportive of your TG friend. The world needs more friends like you, and not just guys. And yes, you're right - friends do have each other's backs, no matter what. I'm not saying that I wouldn't have my friends' backs, or support them, nor would I worry too much that they wouldn't do the same for me.
The difference, though, is that there are more "moving parts," as it were, in a situation with two cd/tg people - which was the general intention of this hypothetical. And now that I've asked the question and digested some of the responses, I realize that the real stumbling block (for me, anyways), would be my own gender exploration that might cause discomfort. It wouldn't be simply adjusting to perceived changes in a friend. It would be that, plus baring a side of myself to a friend with whom the whole thing had heretofore been completely hidden. I don't mean to minimize your experience at all, believe me. But I would have to imagine it would be one thing to watch a guy friend try on a high heel for the first time, and another thing entirely to be asking one another "how do I look in this sundress?"
In any event, great discussion.
(: