Not just cooling. There is a factor in batteries called the Coulomb rate , otherwise known as a 1C rating. That rating is basis of charging in relation to the capacity of the battery. Due to the off board chargers (2.5kW = original charger, 3.3kw R charger) if you ganged them up they would exceed the KW rating of the pack and the 1C ceiling and you would not really see any significant charging difference.
This is why DC fast chargers, superchargers are popular on Teslas and other cars with more...cells/KWs. Those batteries can handle the massive amount of charge which there receiving. Smaller batteries not so much.