K.S.
I can absolutely understand why they would be doing thhe taxs together. That way they can run the numbers using different scenarios to see how to maximize the tax benefits. Also they can double check that they aren't duplicating any deductions which could get them into hot water with the IRS. You should be thankful they are being diligent about this as anyy mistakes on their part could have implications for you after you are married.
If you are this worried about them getting together, I think the 2 of you would benefit from some premarital counseling so these issues can be worked out ahead of time. My husband has coffee with his ex about once a month, he helps her with an occasional project around her house, and she has taken care of our pets in the past. All this and they don't even have kids together. I recognize that they are still friends even though they weren't good together as a married couple. I am secure in his choice to be married to me and not to her. I would never dream of checking his email or text messages or anything like that.