I replaced the single-gang switch box on this basement stairwell stud with a double-gang (shown) so I could add a second switch. I also plan to upgrade the conduit from 1/2 to 3/4” so it’s not overfilled now. My question is - is the bigger box still NEC compliant in this location, given that it’s twice the width of the stud it’s attached to? Thanks!
I tend to agree that, given how exposed it is, it's likely going to become a nuisance and/or get ripped off the stud. There's a couple of ways to play this
These take up a single gang and accept any normal receptacle cover (I would highly recommend going metal for this). It looks like you've got two cables, so break out the tab on the side if it has one.
Two boxes stacked vertically
This might be more trouble than it's worth, but it would still get you a low profile. Buy two single gangs and two EMC connectors. Cut a short length of EMC and connect the bottom of the top box to the top of the bottom one. It's still allowed, and you can use stripped cable to run the extra length.
As noted by others in comments, the problem is that people will bump the switch box on the way up the stairs. Mount it using the left knockout instead of the middle and you should be fine.
If you are, as I suspect, using conduit to provide protection to the cables but using the cables outside of conduit once they get to the ceiling, that's fine. If, however, you are actually running conduit all the way from the panel to this location then you should seriously consider switching to individual wires instead of cable. With metal conduit you don't need the ground wire, so for two switches that's at most 6 wires - and only 5 if the switches are on the same circuit so that they can share neutral. With individual wires (e.g., THHN) you are fine with 1/2" metal conduit. Except: if (as appears to be the case here) you use cables most of the way and then switch to conduit, you can't share neutral wires because the neutral current would not be properly balanced once on the cable portion. On the other hand, if you don't actually need neutral then you can leave it out and add (one or two, as needed) in the conduit section later if needed (one of the advantages of conduit).
A picture in another answer shows a double-switch that is actually a double 3-way switch. It isn't clear what kind of switch you already have and what you are adding. In the past, a 3-way switch could use /3 cable with several different possible wiring schemes. However, current code generally requires neutral in the box and so things get a bit more complicated. Your existing switch should be grandfathered, but for the new switch this could be an issue. In any case, if you use conduit with individual wires then you can't remark white as a traveler or hot wire as you can with cable, you must use white only for neutral. On the other hand, with conduit you don't need to provide neutral if you don't need it, but that may not really apply when conduit is only part of the circuit - i.e., the cable portion would still need to include a neutral wire for new switch boxes, so then you might as well bring the neutral all the way to the switch box.
Evidently some of the folks commenting and answering have never seen how you mount devices in a 4 or 4-11/16 square box. The box appears to be a normal 4 inch square box, probably the 1.5" deep flavor. It's not actually a "2-gang" box at all, those differ in details, but a 4-inch box will accept two yokes.
A plaster ring for two devices:
An exposed work cover for two decora devices:
Exposed work cover for one duplex device:
Two-switch 4-in exposed work cover: