If you are adjusting the amount of interest received because a portion of it is being reported on your wife's return, you would want to enter the adjustment as a positive number and use I received all or part of this interest for someone else as the reason for the adjustment. This will reduce the interest reported on your return. Your wife would need to report whatever portion of it that you do not.
If done correctly, the Interest on 1099-INT reported in the Your Income screen should reflect only your share of the interest.
