It appears that you qualify for a maximum total of $8K credit. Worst case you could be disqualified from both credits (e.g. if you moved into the second home purchased).

Married filing separately also has other negative consequences. Depending on your income, filing seperately could cost you a significant chunk of the credit you are trying to retain.

Consider the second 8K an interest-free loan from the IRS (assuming you didn’t underpay 2009 taxes).

I commend you for trying to maximize your outcome, but hopefully the two purchases make financial sense for you, regardless of the 8K credit.