These things are never straight forward.
When my wife and I separated the best thing my lawyer said to me about our minor child (who is in her teens) was that there was no point in getting a custody order for a 15 year old.
This has proved to be rather true. She seems to do what she likes, and moves from one to the other at will.
There was wise advice from some on the radio, which really said "Don't make the child have to decide loyalty each time they are called to address their parent of step parent"; "Children want adults to be happy and don't want to be the cause of confrontation"
I must admit that it is unlikely any of my children would want to call anyone else "Dad" unless they wanted to really crap me off (and it is quite possible that they might want to do that) but as Freda Briggs, well-respected child advocate put it...the Family Court's time should nto be wasted with this sort of stuff.