How much more Power do I need to run a 4000w sub stage?

[IBleedMusick]

I have decided since I have the Z's I might as well push them like they deserved to be push. I as initially going to run 1 Soundstream TX1.2600 for both but I have decided to just get another one and run 1 to each (gain matched). My electrical for the car when I was running just 1 is 1 275a Ohio Gen alt and 2 Stinger SPP1700's (One up Front, One in the Back). 2 runs of pos and neg Stinger 1/0 to the back. What I need to know now is how many more SPP1700 batts do I need to add to the back to keep my electrical stout? Keep in mind I already own all the amps I just need to buy 1 more 2600 and I would normally sell the 2600 I have and buy 1 big amp but it was a gift from my father and I wouldn't feel right doing that, that is why I am adding a second one (so please no suggestions about running one big amp or insulting the amps I am running). I also already have the 275a alt and 1 SPP 1700. Batteries will be monitored and charged when necessary (1.25a 12v Battery Tender).