Yeah it's pretty simple, you can't use two of them with your MBox : Not with SPDIF anyway. The Alpha Channel has analog outputs right? You can always connect those to your MBox's two channels.
Really though I would say get something else like an API A2D (it's about the price of two Alpha Channels
http://www.sweetwater.com/store/detail/APIA2D/), two 3124 preamps with SPDIF output. IMO the 3124 pres will be just as good, if not better than the one in the AC. Also, the Alpha Channel is really just a preamp and EQ... neither of which are really specifically derived from any of the well known/sought-after SSL stuff. The limiter that's built in lacks the parameters you would want to have for it to be useful on anything like vocals, snare, bass, etc. Personally I'd rather EQ ITB than waste money on the SSL, but that's just my 2 cents.