Yes copy one to be the primary person on the Joint return. Like open your MFS return and go to FILE - SAVE AS and give it another name. Then change it to Married filing together under Personal Info. Then add all her info to it.
If you had done the Joint return first (or maybe it works on a separate return too) you can do a What-If worksheet. Go to Forms Mode (click Forms at the top) and click Open Form. The What-if form is near the bottom of the US1040 listing (you have to hunt for it). You can play with that.
