can I share the same neutral on two separate 20 amp circuits
If your heaters are 120V then it's possible to use a shared neutral for the heater circuit and the receptacle circuit, as long as you follow the code rules that apply for this type of circuit.  (If the heaters are 240V, there is no neutral on that circuit, and they'd have to be wired separately.)  

If you haven't already done so you might want to consider whether you're better off with 240V heaters and separate wiring.  Even if I didn't want 240V now, I'd wire these separately so that I had the option of converting to 204V in the future, rather than save a little wire now with the shared neutral.