Go ahead and run the two Deka's in the back with the stock Honda if you got the room and enough to make it look nice...also doing so will cause less voltage drops....
Good luck
this is not necessarily true. if his old battery is shot and cant be revived its not really going to benefit any from it.
This. If the front battery is dead and you hook it up with the 2 Dekas, the voltage would try to level out between the 3 batteries. If the stock battery is "shot" (like N8 said), the two Dekas would also suffer because their resting voltages would be lower in most cases (due to the single bad battery).
I hope this made sense to you