Archie,
I know what you are feeling. Love the look since I was a little kid. That said, whenever I do them, there are at least a few complaints of having to wait for the other group, aka, slow play. I usually only share the back tee, that no one plays, so I can't really think its an issue.
One of the beauties of shared proximity tees at least, is one bathroom can service two holes! If you can make one front nine and the other back nine, so much the better, although, I have heard complaints of "slow pee" when I do that, so its always something......