Both of the batteries must have the same mAh capacity rating.
**broken link removed**
Maybe with this quick schematic you can see why. Imagine that battery 2 runs out of power since it has a lower mAh rating. Battery 1 it still pumping current, and to get to the players, it must go through Battery 2. What ends up happening is that Battery 2 will get reverse charged, causing a lot of stress on the battery and a probable fire... When creating battery banks in parallel, I always am sure to use the same battery brand and model.