So it turns out Pub 915 says you can net his negative against her positive.  And that still leaves you with a negative?  If it's under $3,000 you're out of luck.  If more, you can claim it on Schedule A, Line 16, or go through the computations for the Section 1341 credit.  

But it's the net Box 5 amount that you deduct.  Not Box 4.