[Alan Anderson]

The best way to put two used robot batteries in parallel is...not to do it in the first place.

If you have two new and well-matched AGM batteries, and take care to always keep them discharged and charged together, you can get away with it and not expect any problems. But since you'll probably be running them down pretty far in typical operation, any difference in their capacity will show up and tend to drain one to the point where it might not recover.

If you can live with the slight voltage drop, you can put a high-current diode in series with each battery before tying them together. This will prevent the potentially harmful situation of one battery stressing out trying to keep the other charged.

My preference would be not to use otherwise valuable FRC batteries at all, and instead use a motorcycle or deep-cycle marine battery. The only real drawback I can think of would be the need to keep from tipping the battery on its side.