AFAIK, you can't use both M-Boxes at the same time. I thought the M-Box Pro could handle 8 total inputs(5&6 being the RCA unbalanced inputs and 7&8 via the SPDIF input. If this is something you rarely need, maybe borrow/rent an interface, or buy something from Sweetwater or anyplace that has a good return policy. Alternately, I would likely sell both Mboxes and buy an interface that covers your needs better(and fits the kind of connections you have available on your computer). If you have a Thunderbolt port, that connection will probably give the best performance(Presonus and Focusrite make some good TB interfaces)
