This is what I was told as well. More specifically, one should be running a single battery at about 80% max of continuous discharge rating. We add a little fudge factor with dual
batteries run in parallel because we have to assume some slight imbalance (i.e. you're actually drawing a little more from one battery or the other, or one battery can unload a little faster, or your contacts aren't in exxxxxxactly the same condition, etc). So, we drop that 80% to 75% for fudge. (2 X 20A
batteries = 40 . . . 40 x .75 = 30A)
Also too, it's good to check battery voltage before charging, to make sure they're balanced. (I actually take a couple three nice big taps on my dual mod before pulling the batteries, and I throw them in the charger straight away .. . . it reads voltage.)* You also want to see them charging at the same rate. If the voltage reading after use is more than about .02V off, maybe get a new pair of batteries.
Speaking of pairs. One must pair up a couple new batteries and treat them as a set from then on. You're basically making a "battery pack".
I'm not the Battery Pope, mind you, just some of what I've picked up along the way.
*Batteries in a parallel config can balance each other out. One will basically charge the other in an uncontrolled fashion (i.e. rapid discharge/charge). That's not good. This is why I give the batts a little run before pulling and checking voltage right away. Also, this is, in part, why it's important to start with an equal charge when putting in fresh batts and keep 'em paired and use batteries that are new or have exact equal history, etc. etc.