2 x 6000 will be notably heavier than a single 10000, and redundacy failure is a small risk - I've never heard of a cell failure in the air. You're likely to introduce chances of failure with double the connectors, a Y cable etc.
I have 2 x maxamp 10900's, and they are *expensive*. But good! I fly a single pack, then another. I rarely need more than this, but then again I'm not doing this in a professional capacity.
However, I've recently started using 8000 packs from HK/Zippy, because they're $110. Not cheap, but definitely affordable. And on my rig, the weight reduction does compensate a little for the amp loss, so I don't lose as much flight time as expected, and a bit of trim compensates easily for the CoG change. And my charger can charge 2 at once (1C), where it would pop out (the house fuse!) doing 2x10900. I don't charge in the field or go beyond 1C/balanced.
I see foxtech have 16000 "dual core" packs out now, for a - and I say this loosely - "sensible" price. Will be interesting to see how they perform. Pity they're the wrong dimensions for my rig - I'd need an "SBS" config for that pack.