It occurs to you that when Tiffany asked Mitch to come over, she didn't notice anything was strange or think that she was being made to do anything. She thought everything was natural.
So what about your change?
You walk out. "Tiffany, do you think I look better or worse than you in this?" you ask.
"What? Oh, come on, Kelly, we're twins, we both look exactly alike."
Which means your guess was right. There wasn't any need to fix the two Tiffanys problem, because not only do you look just like her, it became normal. She thinks you're supposed to look like her. You're probably real siblings now. So you can wait for Mitch to arrive and nothing will seem wrong.