The answer is yes you can have 2 separate circuits in the same box (they can have a splice also but not needed in your case). The only concern would be the total box fill. Based on the NEC the wire fill would be calculated at 2.0 for each conductor for 14 gauge wire and 2.25 for 12 gauge wire. The device or switch counts as 2X for the largest wire connected to it or each in the box. All the ground wires count as 1X of the largest ground in the box. If the box has clamps they are counted as 1X of the largest wire. With the numbers added up they need to be smaller than the listing on the box for cubic inches. If more information is provided on the size of the wires entering and exiting the box and total number of devices we can help with the calculation.
I would use drawn steel boxes and EMT conduit, it's bulletproof, firesafe, cheap, easy to disassemble to alter or access, and grounded.
I thought it was not cheap and I was mindlessly gold-plating everything, until I looked at the prices for the blue cheese. Holy smokes, it's more than steel boxes! And the orange backless are even more! Why...
And I find EMT easy to work with, just cut it with a hacksaw and a few spins of your wire stripper deburrs the inside. And it all spins together either with compression fittings or setscrews. No cementing.
On the original light switch, you fit a "blank" steel cover plate with a 1/2" knockout on it, and extend the EMT off that.
If the underlying box is steel, and the cover plate lands hard on clean unrusty unpainted steel (not held above it by drywall etc.) then the cover plate and extended EMT is a valid grounding path. Otherwise you will need to extend the ground in a wire. Either a bare solid wire or I prefer a green stranded wire.
In conduit, you are able to use stranded individual wires (THHN) which is easier to work with, but a little harder to attach to devices (and the device must be listed for stranded wire, can't say "solid wire only".) I like the Leviton Screw-and-clamp types where tightening the screw clamps the wire.
However in conduit with individual THHN wires, you are not allowed to use white for a hot wire. That is relevant in "switch loops", just use two blacks.
Best Answer
You can have multiple circuits in the same box so long as you do not overload the volume of the box with devices and wires.
See this answer by @Tester101 that discusses box volume.