My wife just file her taxes. now how do I open a new return

you can't use her account unless you want to permanently delete her return. you need your own account but can use the same e-mail address for both. not sure why you doing married filing separate returns because that almost always results in paying more in total taxes on the two returns than on a joint return. certain credits are lost, certain deductions are limited or eliminated and both must either itemize or use the standard deduction. 

with the desktop version (full PC or MAC)  for one price, you can prepare 3 returns to see the effects. one for each of you as married filing separately and then a joint return.