Well my Tiger 1 tanks in the hot FL sun, sand, and high grass I get around 30+ min on my 5000mAh batteries, but then again I don't shoot much airsoft either and drive slowly for the most part. I still think 1 good battery is better than two OK or less than that batteries. Putting them in parallel is not that great of an idea IMO. If your current battery cant handle it then getting a new one will not help the situation. In fact you could potentially have a safety issue if the batteries are too far off when mixing them, charging is going to be a pain due to twice as much heat, need thicker wires to run more charge current, etc. Honestly a single good battery cost less than two and all the accessories and such needed for running them parallel.
Also just because a battery is new doesn't mean it can't be bad. I just got 20ish packs from a manufacturer and 2 of them had dead cells in them. 6 cell lipos are NOT cheap either. These are name brand packs too, not your average cheapies. They measured fine but once I pulled some current from them the cells just died. Have you tried to put your battery on a smart charger and discharge it and charge it back up and see what mAh you are getting out of a 1C charge/discharge cycle?