Short answer: You cant
Real answer: Lookup CGMiner-Dmaxl, compile and run that on your RPi B+ on Raspbian, works for me no problems to Dual Mine on 10x Gridseed Orbs
Take a look in the Minera Thread in the Software section, I outlined exactly what the startup commands and order is to ensure it works.
Wow that sucks that BFGMiner can't do both, I thought at least it would require two commands like CGMiner. I will checkout the thread that you speak of, I greatly appreciate the help