What would be the easiest, quickiest and dirtiest way to communicate both? In two scenarios
1- unidirectional. Sending data from arduino to the Bela (just to send a number of "data channels" and their values, from data captured through an arduino analog&digital pins)
2- bidirectionally
I would eventually do it with the multiplexer capelet, but some sensors do require switching the analog inputs to voltage sources (for example, for resistive touch screens) and I don't think that could be done.