agreed,
unless its a really high "C" rated battery.
I mean, a 50-100 "C" rated 2000mah pack might be OK. Even then, you risk puffing up the pack.

It would likely be easier to swap to a system made to handle more voltage and 1 battery.
You dont have to run duel packs. There are 6,8 or 10 cell lipo packs.

Even tho that means some $$$, you wont be risking burning up or damaging your existing lipos/esc.