Been flying with Lipo's for 6-7 years now (I was a pretty early adopter). I have flown and charged packs in 2P configuration 100's of times with no issues. With that being said it is safer to use two of the same packs. I do not own a ballencer and so far that has not been a issue but I do keep an eye on individual cell voltage with a DVM. I also try to charge all my packs at .5C. Been doing this for a few years and it really seems to help with the cell longevity, as does not pulling every last MAH out of them. I have 4 TP2100PL packs that get abused in a Steven Aero Edge with a Mega 16/15/3 that pulls 39A WOT and the cells are all closing in on 100 cycles and still going strong. YMMV but I have had great luck with Lipo's in 2P configuration.

Cheers,
Jim