Depends on your application. How many shots you going to take?
In my experience, 2x batts in the grip will last a day and a half. (1000++ shots)
And yes, there is a duo charger: AC-VQ900AM
(And I assume the free batt is original Sony batt?)
BTW, the dual battery charger isn't very good... it's a sequential charger apparently, not a parallel charger... so it means one battery gets charged at a time... the switch over to the next battery is automatic... that's about the only advantage... for me I would just get another ordinary battery charger and I can get two batteries charged at the same time... faster, lighter and definitely cheaper?