Hi Matt,
Never having an active weapon I cannot suggest from experience, however from a personal prospective, I would go with two separate batteries. Two reasons: 1. you can tune the battery to the system requirements, 2. should one system have a problem the other will remain healthy.
As for the charger, you can pick up chargers on ebay quite a bit cheaper that will do the job - try looking at an iMax B6 - these can be bought with an integrated charger or separate.

Hope this helps, regards Cliff