[Patrick]

For packs of different cell counts (2s + 3s) you have to charge in series, you can't connect packs of different voltage together in parallel. The guys says that about 15 seconds into the video as well.

I don't think the average charger and balancer setup can do everything that the Xtrema setup in the video will do, so it's easier to charge in series usually. You could charge the packs in parallel, but as far as I know you wouldn't be able to balance every pack at the same time like they did.