You have to remember if your drives both will be putting out 2000mA each then each one will be drawing 3000mA or around there so I would use no smaller than a 26650 cell or cells and I think 32650 would be nice if
a good host can be found that is to your liking. The SK can take a 32650 if I put it in the lathe and take out about 0.25mm of aluminum from the inside.
The two XB-2 drives I sent you are the very latest model they are capable of 4000mA each so with the proper heat sinking they may work for you.
Do not try and run them above 2000mA with out heat sinking them, I do believe they have a built-in temp controller so if they get too hot they will shut down till they cool off, I'll check on that to be sure.
The full back side needs to be in contact with a heat sink if your going to take one all the way to it's max current of 4000mA !