Your two families know that you are now married, and they can't both expect to have your 100% holiday attention.
You might see one mom the night before Mother's Day, the other on Mother's Day, or alternate years, or alternate holidays. Maybe "hit" Mother's Day for one family and "hit" Father's Day for the other family.
After all, your own parents had to figure this out when they got married. If they are reasonable people, they'll know the drill.