If you research a little, you'll find some discussions about back-to-back toilet problems when the proper configuration is not done. Today's toilets flush MUCH faster than the old ones, and the waste from one can skip over into the opposite one unless the layout is not done VERY carefully. You need to also consider that the carriers for these toilets, especially back-to-back would require the wall to be VERY thick, and take up a bunch of room for your two small bathrooms. So, could it be done? WIth a thick enough wall, yes. Is it really practical? Maybe not. Would tearing up the concrete and replacing the fittings for two toilets work out probably better? I think so. Be interesting to see what I may have missed in other (hopefully) commenters thoughts.