Talk to your attorney AND your tax accountant about this. If it benefits you (by negating his claim of when the marriage ended) then file jointly. BUT - you would likely come out ahead if you filed separately. You might negotiate some agreement with him for him to pay you half the difference of the refund you would get if you filed separately. Since it hasn't been negotiated yet who gets to claim the kids on their taxes, it might be easier to file jointly, but you want to make sure he doesn't steal the whole refund (if there is one).
Btw, I'm no lawyer, but it seems to me it would be pretty difficult to claim the marriage ended 2 years ago if you've continued to file jointly as married AND been living in the same house???
