Is claiming both my children in my tax return illegal if I did it because of a verbal agreement?

I claimed both my children on my tax return because we verbally agreed to share the tax returns. There isn't anything in our decree stating who gets to claim the kids. Now, she denies having a verbal agreement. She claimed both last year so I feel I deserved to claim both this year to be fair. Now she is taking me to court. Did I do that illegally?