Best way to get the packs synchronized is to simply charge them up individuallly, then connect them together. After that, its no problem to charge them together.

There are a set of circumstances where adding a pack in parallel will overload the ESC, but it's highly unlikely unless you just cobbed your power system together from random parts. The circumstances are if you're drawing on the single pack so hard that its voltage is dropping signfigantly, AND the Amps are right at the ESC's limit. When you add a pack in parallel, it shares the load. The lighter load on each pack allows the packs to maintain a higher voltage. The higher voltage causes a higher current. The higher current is more than the ESC can handle....