Struggling to get my two dogs to stop fighting

I have a 2 year old golden doodle (Maisie) and a 4 month old yorkie bichon (Murphy) and they never stop fighting. Maisie bites Murphy’s neck and pins him down and Murphy bites Maisie’s ears and mouth very hard. I don’t think it’s play fighting. What should I do to make them less aggressive with each other/teach them a better way to play? They both instigate it and they sometimes seem to be having fun but then it elevates to too aggressive and I’m worried one or both of them is gonna get hurt especially because of their size difference. We separate them if it seems to be getting to aggressive but I would like them to become civil with each other.