12 cells has nothing to do with it.
2 2000mah 6-cell packs in parallel will appear to the charger as if it were a 6-cell 4000 mah pack, which would take twice as long to charge as a single 2000mah pack.

Overall, the recommendation with parallel packs is to separate them for charging, to ensure that each pack is properly charged to it's peak. You could use a switch or unplug the y-harness.

*If* the packs are charged in parallel, there's a fair chance that one pack will peak before the other, and the charge may detect this false peak and stop charging, leaving one pack mostly charged but not fully charged. In the *worse* case, the charger might miss the peak of the first pack if the other pack is still soaking up current, and end up over-charging one pack.

I wonder if it's more of an issue with high-performance vechicles (cars, planes, boats) than with tanks, which generally aren't sucking a pack dry in 5 minutes of racing. You might be able to get away with it, but I'd avoid fast charging the packs in parallel. Use more moderate charge rates, and check the temperature of the two packs. If they're similar, you're probably okay. If one gets really hot, it's being overcharged. If one stays really cool and never gets a hint of warmth, it's probably being undercharged.

D.

D.