you'd gain nothing, but run the risk of unbalancing the pair. In theory yes it can be done.
Assuming packs are 5000mah charging at 5A will take an hour, but your setup makes the pack 10000mah. to charge at 5A will take double. 2 hours.
You'd be better off buying this
http://www.jespares.com/electric-mod...ategory_id=486