Have ridden a prototype tandem with Gates belt crossover a couple years ago (before Co-Motion or Calfee had it). Was not overly impressed.
A well lubed/adjusted cross-over chain is not noisy.
As for weight weenies, you can use smaller cross-over chainrings and . . . less chain too.
As for no chain tattoo/grease marks, we don't get that as we lubricate our chains via the hot wax method.
Maybe am a bit of retro grouch? We still use barcons (yes tried D/A STI) and we use toeclips/pedals (yes tried "clipless" . . . you still got to 'clip-in').
If you got the need for bling and the dough . . . why not?!
Just our input.
Pedal on TWOgether!
Rudy and Kay/zonatandem