Has anyone made any rulings for two players playing one two-headed character?

Hi there, I'm wondering if anyone has any ideas for how you'd rule two players playing one two-headed player character. It's something some players at my table want to do and I'm open to the idea, I just can't think of how I'd rule it. I was thinking maybe dividing the actions into each head gets one action per turn in combat unless they're doing something that required both hands and then for casting they'd have shared or maybe 50% extra spell slots instead of double and no extra movement? I haven't thought about it super seriously which is why I'm looking for ideas.