This is an issue with the new Ultegra tandem cranks - the timing-side chainline is a couple mm less than on the FSA cranks (51mm instead of 53mm IIRC). I've been running a different set of captain's cranks (the FSA cranks are too long for me) with some small spacers (2mm I believe) behind the belt ring for a while now without any issue, and I'm not worried about it, so that is what I would do here.

As for flipping the belt ring over, then it should work just fine. In the normal orientation, the lip of the ring does provide some protection to stop things getting caught between the belt and ring, and you will not have that feature, but neither do the center track versions of the belt drive, so it's obviously not essential. On our setup, we get very odd noises and the belt creeps around a bit if we install it reversed to normal; other people don't seem to have that problem, but do try flipping the belt if it is making noise or not staying aligned.