Basically, you don't. When filing a joint return there is no such thing as "my" refund. It's only "our" refund. But for the debt there is only "his/her" debt. So when filing injured spouse because one spouse owes a debt and has a lien against their tax refund, that does not mean it's a 50/50 split. It's based on how much each individual spouse actually paid or had withheld in taxes.
For example, if both persons each earned $20K individually each year while person A has $500 coming back and person B has $5000 coming back based on the with holding from each individual's pay, then person B overpaid their taxes more. If person B is the one with the debt then the entire $5000 is applied to the debt and only the $500 of person A is refunded.
