The weak link in these the internal BEC in the esc - overheats and fails and then there's no power to the esc. You can run an external BEC and run higher than rated input pretty reliably. I ran 4S and 5S with my MM using an external BEC for a while and never had any probs doing it.

I don't see where 2 6 cell NiMh pacs in series should cause probs, as soon as you hit throttle, voltage will be under 15v and since you're running 4600, its within specs. There is no diff in two 6 cell pacs wired together in series and one 12 cell pac from a voltage standpoint.