I got 2 2200 Lipos connected with a T-cable and little or no modding. Just a notch out of the battery door on both sides. And I made battery holders from trunking rails, the ones you use to tidy up cables along the wall in your house. Attached them to the motor arms with tie wraps. Very simple, cheap and it works flawlessly. Once I was satisfied with it I attached them permanently with little parkers.
My FC40 is about 1300grams with Gopro, FC40cam, 2 batteries. I got 13 minutes before it hits 11.1 Volts at which I set the Lipo warning buzzer. It will fly for another minute and a half before it auto lands at 10.5 volts. I guess with 2 2600 batteries I might get 16-18 minutes.
The battery rails allow for fine adjustment of balance by sliding the batteries forward or backward, allowing you to take off with just the throttle stick input, no matter how your particular weight distribution is. Also, without any further modding it allows you to experiment with different batteries. It holds a lot of sizes of batteries this way.
If you're interested in the DIY battery rails let me know, I'll post some pictures.