Paul, I use CB200 fed from two Jeti Max BEC linear regulators, yes the current drawn is not equal but it isn't all taken from one battery. Both batteries are used, the proportion is about 2:1. Not ideal but not as bad as one battery only at a time that you fear.
I have never tried it but everyone says that A123s are okay to use unregulated.
Or how about a Jeti Max BEC 2D which takes two batteries as inputs and does the balancing between the batteries. It has one output which the CB200 will be quite happy with.
Last edited by HarryC; 08-04-2015 at 03:54 AM.