I agree with everyone else. It is impossible that the two will stay aligned when longarm quilting due to rolling up on the takeup bar.
I would refuse to make it a two sided quilt, but would instead agree to make two quilts from the two tops. I understand that would cost her twice as much for longarm quilting two quilts, but there's a good reason what she wants to do is seldom done.
If she really is insistent on making this two sided quilt, she should baste and hand quilt it. Just mho.