Apparently Wisconsin, a newcomer to community property, is still backwards in figuring out how to avoid these problems.  In other states, the community ends when the divorce petition is filed, which probably would have avoided the situation here.  

If anything goes wrong with either return, you're the one who will be blamed.  That's why I usually tell both spouses to go find another preparer.  Even if you do the split correctly, don't expect IRS to follow it.  My happily-married clients who filed MFS last year to qualify for the unemployment exclusion, are still dealing with it.  (It saves $4,000.)  Wife's return was processed but she was not given credit for half the husband's withholding.  Husband's return is still being processed.