<font color="blue"> Q. Is it alright to share one neutral and ground between two separate circuits?
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Are the breakers on the same line(120 line to neutral), or are they on each line (220 line to line)?

If they are on the same line the neutral current is the sum of the line currents and you can over rate the neutral line. If they are on each line the neutral current is the difference of the line currents and the neutral line is within rating.

I wouldn't do it run 12-2 it's cheap.