Since there is no settings in software to ‘choose’ one over the other, I have to assume it works with just one at a time. You could run a quick test, though, and potentially prove me wrong. It would be interesting to see which gets priority if both were being fed concurrently in your proposed situation.

What am I thinking? Must need more caffeine.

There is only a single input to the Beam. In order to use optical, you have to use the supplied Sonos adapter, which takes the optical signal and changes it to ARC. With a single input, there’s no possibility of the Beam being able to differentiate. 

Thank you for your reply.