I have a large mirror that's glued to the wall so I can't remove it. It already has a 1 gang outlet in it but I'd like to expand it to a 2 gang box. How would I expand the hole on a vertical mirror that also has drywall behind it?
Get a 100mm angle grinder and stick a smooth 100mm diamond wheel on it, you should be able to plunge cut the slots needed to remove the extra piece of glass
wear a dust mask, ear and eye protection, go slowly brace against the wall pr mirrior with your elbows and use both hands to steady grinder keep the blade straight when cutting.
wipe down all surfaces with a damp rag when done.
Cutting or drilling a hole in a mirror is something that takes either very specialized tools that are not easily available, or an EXTREME amount of luck. If you have a high speed rotary tool, like a Dremel, and a diamond encrusted bit made for grinding glass, you might get lucky. Do NOT breath the dust that comes off of that.