Isolation circuits (back up) to draw off the strong pack is not a very good concept. As the pack under load is always going to be weaker than the pack not under load. Thus constant switching is done back and forth between the two packs. Sometimes with dire consequences, especially if the timing threshold is slightly too long, and some of your electronics (ecu, gyro, etc...) may have to re-initialize.

I've seen this to cause engines to flame out. The ecu didn't like it.