Merry Christmas, Folks...
The two hots on the same phase would not be good. The neutral would have the possibility of having twice the current.. i.e. 40A if on two 20A breakers... just like paralleling the circuit. I suppose one could size the neutral for 40A (#8 Thhn Cu) and have 20A Breakers, but this would prove expensive, unusual, and probably a violation of code...(help me here guys...)
The ideal is to balance the loads on the phases to cancel each other out and provide no neutral current at all. The neutral only being there for unbalanced "line to ground" loads. Hope this helps...
