can I change single switch to combo switch

A reason for the 'current in a set of wires to sum to zero' is to avoid magnetic effects when the wires are in a metal raceway, or when the wires enter a metal box. If a circuit is split between 2 romexes, both romexes should enter a metal box through the same knockout. (Also true for K&T.) Plastic boxes aren't a problem.

(For a service or feeder with paralleled conductors, you can sometimes run separate plastic pipes for each phase. There are additional requirements for where the pipes enter a metal box and for ground wires.)

The basic rule is 300.3-B and 300.3-B-3.

Thanks for posting the link. I'll check it out.

Do you happen to know where in the code the part about having a neutral at the switch is cited? I can search and try to find it, but if you know which section I should look at, that would help.

That will be in article 404.2(C) of the 2011 code.

Excellent. Thanks! I just read it. I think it also answers a question I had about 3-way switches. It looks like there is an exception for 3-way switches where the switch is wired as a switch loop.