Generally, for parallel charging, you'd want the packs to be pretty close. Also, don't forget that you'll also need to come up with a parallel balance harness to balance them at the same time.
The series charging arrangement is easier (IMO) because the cells in each pack are treated just like if they were one big pack. You just need the external Y adaptor for balancing/charging to mimic what would normally be hard-wired in a single pack.
Since you are using two 2s packs, this diagram from my site would be the one to use: