I receently had a new 400 amp service installed so I could have enough circuits to do some remodleing. I am in the process of pulling a 12/2 for the gfi outlets another 12/2 for the heat and a 14/2 for the fan light. The feeds for these will go to the switches and then to the combo. The reason I am doing this is because the old combo on the oldcircuits would dim when we had the heat and light on. Can I leagaly run 2 separate circuits to the new combo to achieve this? Also if I put in a combo with a night light can I run a 12/3 and a nother switch to control the night light off the fan light circuit? P.S. Thanks a bunch in advance!

Yes you can run two circuits to the heat-fan-light combo, as the heat needs a separate circuit.

Usually I have run one 20 amp circuit just for the heat-fan-light, and another circuit for any other lighting in the bathroom, along with a 20 amp circuit for the GFI receptacle(s) in the bathroom.

If you run two circuits to the combo unit, be sure to keep the neutrals for each circuit dedicated to that circuit and load.

I am with TK on this. I have never had to run two separate circuits to a H/F/L unit. In fact most say to use a dedicated 15 or 20 amp circuit to the unit, depending on how big the heater is.