I wouldn't recommend using the 2500mAh packs but the 3300mAh or 3700mAh packs in series should work fine.

The advantage of using a single 6s pack allows you to recharge it in the plane without needing more than a single MPI arming switch to disconnect the battery from the ESC. This will be demonstrated later in my review using a safe balancing charger like the FMA Cellpro 10S.

When using two packs in series, you must either charge only one pack at a time or break the series link between the packs in order to charge them at the same time using two chargers. If you don't do this, then a ground loop will exist and damage one or both chargers. In essence, you need to use two of the MPI arming switches or have some access hatch to disconnect the Dean's Ultra connectors; one from the ESC and one from the other pack in series

On smaller scale planes like this Ultimate 40, it is often difficult to remove the lower wing to recharge during flights. I have found it much easier to assemble the plane once for the weekend and recharge the battery inside the plane.