If you're willing to consider them truly separate keyboards when connecting them separately, it might be practical. But DOX specifically said he'd use at most 1 Teensy and 1 I/O expander.
On the plus side, if DOX changes his mind, treating the 2nd Teensy as an I/O expander until proper dual Teensy firmware could be written should be fairly trivial. Or if he doesn't, I suppose one could always buy another Teensy, put it inside an Altoids can with a few connectors (which I think looks really cool, lol), and use that as the right hand side to save desk space when they only want to use the left.