Easiest way would be to replace one of the switches with a duplex switch (two switches that occupy the space of one).
I would disagree.
A. It takes only a few minutes to remove a nail on box.
B. You have tons of room to reach into the wall cavity to pull your new wire.
C. You end up with a box big enough for full sized dimmers.
1. Turn off power
2. Make sure there is space for a 3G box by probing with a long skinny screwdriver.
3. Disconnect wiring in box (make note of switch legs/configurations) If plastic, ue needle nose pliers to pull the built in clams back so the cables will pull out easily.
4. Place your 3G box over the existing 2G and mark it
5. Use a drywall saw to carefully cut just
above and below the existing box and extend to the 3G width.
6. You may be able to grab the nail heads with your dikes via the larger opening and work it out. If not, use a screwdriver on the stud side of the box to pry it loose. The nails may be installed at an up/down angle (blue box) so the top and bottom ones tend to pinch it in. Once both are a little bit loose, pry out one at a time. Worst case, you can cut the nails with a sawsall but it is really easy to slip and hack the drywall.
Crap....I coulda done it by now