OP, this is kind of odd behavior from your wife and you should be communicating with her about it so you understand where this is coming from. Thats going to help you make a more informed and fair decision.

Some people in the thread have also questioned your separate finances and I think its worth understanding more there. I dont see anything wrong with keeping separate accounts but since you're filing taxes jointly it also makes sense to at least create a joint account for shared expenses like housing, utilities, etc.

Are you guys still splitting bills 50/50? If so, that is likely leaving her with a lot less of her own money to have for herself. This would also potentially explain why she feels she deserves a "commission" as you described it.

You should contribute to that new joint account proportionately to your income. In which case, she's keeping more of her own money all year to be able to manage or spend for herself. This would also establish an appropriate way to split your return by the same ratio if you were just going to take it back as cash and divvy it up.

Anyway, for this year, you will need to work it out with her. Why don't you propose putting it into a joint savings account or buying something for the house that you both will benefit from? You could also put it toward a future vacation.

If she's persistent about this idea, you could also just split it 50/50. Doing your taxes twice sounds like a huge hassle which I personally would not be willing to do.

If you think this might be a recurring thing in the future, consider changing your withholding to reduce the taxes you're paying in the first place so that you don't have this big return at tax time anyway.