How do I mount wall switches on opposite sides of a partition?

If you want switches back to back I would recommend using a 4 square metal box with a 1/2 inch single gang mud ring.

There are 4sq boxes that have side mount brackets on them. They are easy to use.

Run a Decorative Light of a Power Outlet that you can reach at Night. Or you can buy an Infra Red Switch that will turn on the light when someone enters the room. Ask your friendly Electrician to look at the problem. There is usually an easy way ( for him ).

It seems as if you are talking about two finished wall surfaces here.

What you need are called "old-work" or "cut-in" boxes; I'd use metal boxes if you are concerned about box fill issues. Use the type with the built-in expansion wings; they stay in place better than Madison hangers. The downside of any plastic box, or these old-work boxes, is that they are not generally used with a device ring, which means that there is no additional box volume; you only get what the box itself is rated for.

The solution to that problem is to use deeper boxes, and not install them exactly back-to-back.

Because you are installing Type NM Cable, you start at a disadvantage as far as box fill goes. You must splice the cable in each box it passes through, thus doubling your fill count for those conductors. For any switching application, you need to count a minimum of 7 conductors in each box [hot and neutral in (2), switch leg and neutral out (2), switch (a double count for each yoke mounted device - 2 more), and the equipment grounding conductor (1)]; if the box has integral connectors, you must add one more conductor volume, for a total of 8. To meet the fill requirements for 8 #14 AWG conductors, using standard sized boxes per NEC Table 316.10(A), the smallest Gem Box (the street term around here for "cut-in" boxes) is 3 x 2 x 3-1/2". That is the largest Gem Box available as a "standard" size, and its allowable fill is 18 cubic inches. Since any #14 AWG conductor is assigned a volume of 2 cubic inches, per Table 316.10(B); the 8 "counted" conductors total 16 cubic inches. If you are using this box as a junction or splice box for your #12 conductors, it won't work. What you can do, is to "double up" the boxes by removing a side cover from each box, thus doubling the allowable fill volume. For this, you may have to use the Madison hangers.

As for the 20 Amp 3-way; you have added an additional conductor, by means of the traveler. You now have a hot & a neutral coming into the box (2), a neutral leaving the box (1), two travelers leaving the box (2), one equipment grounding conductor (1), the switch (2), and integral clamp (1). That now gives you a total of 9 #12 AWG conductors. Given their assigned volume from Table 316.10(B) of 2.25 cubic inches, you will need a box with a volume of 20.25 cubic inches.

In your position, I would purchase 4 cut-in boxes 3 x 2 x 2-1/2"; and "double gang" each pair. I would also opt for purchasing the boxes without integral clamps, and use listed Type NM cable box connectors. These take up no volume, so you save 2 & 2,25 cubic inches per box, respectively. You can purchase multi-part face plates that snap together to cover the half of the box assembly that contains no device. Some manufacturers even make covers in that configuration; but they are never found at home centers, and even electrical supply houses may have to special order them. This would almost certainly be the case when using the Decora type devices.

Feed both sets of boxes separately (if you have access for wiring one box, I am assuming you have access for wiring each of the separate boxes). If you must feed through the one set of boxes to reach the other; use the 3-1/2" deep boxes where you have to splice the feed-through conductors. Just remember, that if you are splicing the #12s in that box, you must count the grounding conductor from that cable, not the #14. It only adds .25 cubic inches, but I am always very careful about box fill, and I always count each box when I inspect new construction wired with Type NM, because, unlike a conduit installation, I can do it without taking any wires out of the box.

You can offset the boxes on opposite side of the stud cavity, and run a length of NM between the boxes, and not have to worry about the boxes combined depth being too great, as you would in a back-to-back installation, and not have to fit a close nipple between the 2 boxes.

Remember also, if you do screw or nail any boxes to a stud, through the inside of the box, the fastener must be located in the back 25% of the box.

Sorry about the long answer, but I feel if I am going to give you an answer, you deserve an explanation, as well..

Good luck.