I have done this. Before you begin you need to determine a few things. A) is the wall structural (you say it isn't). B) is there any electricity or plumbing in the way. You're going to need to rip 2 x 6's down to 4" since you have dimensional lumber in your walls. Begin by outlining the opening location for the new cabinet. Extend the lines well beyond the actual size for later reference. Then, remove a section of the wall - a square cut extending half-way into studs which will remain. Cut any conflicting studs 1.5" above/below the cabinet outline openings (give yourself a 1/8" additional overall). Install level horizontal 2 x 4's anchored to remaining studs. Then install vertical 2 x 4's for the width (again allow an extra 1/8"). Dry-fit the new cabinet, remove and using one piece, fill the entire opening with new drywall (since your walls are plaster you may need to "skim coat" to final thickness since drywall thickness may not match). When finished, follow your extended lines to locate and cut-out the opening for the medicine cabinet. You may wish to paint before permanently installing the cabinet. If the wall is structural, it will be necessary to install headers (doubled 2 x 4's) above and below. I actually slid 2 x 4's against remaining studs, anchored them and attached my header atop and below their ends and onto the cut end of the removed stud.

Here's the one in my bathroom in Cathedral City It's at the far left in this photo, and the sink I use is on the right. This medicine cabinet is empty - I just never thought to use since, since I use the sink on the right, which is closer to the towel racks, which I consider to be more important. This medicine cabinet has a mirror on the inside of the door. My medicine cabinet in L.A. is pretty much full, and I would like to replace it with a new one. When I redid my brother's bathroom, I kept his like-new medicine cabinet and had to make sure that the door swing would clear the wall mirror that I selected for him. There is about 1" clearance:

I've seen his and her toilets facing each other in a bathroom. On platforms. I could never quite figure out whether they just wanted their very own and were never in the bathroom at the same time or whether there was something else going on. In any case, I never asked. I kind of see the point of something that is accessible on both sides, actually you would only need one of a number of things since it would be available to both bathrooms. I actually played with this idea briefly because the medicine cabinet in the hall bathroom is in the 6" deep by 20" wide stud cavity over the in-wall-tank toilet for the ensuite bathroom. So I did think...what if this was accessible from both sides? And in a house with no kids, I doubt anyone would use it to poke through into the other room or anything. And I have heard people in this house use the bathroom. I hear people using the bathroom at work. People go to the bathroom. Maybe you are going to hear it. I dunno. I try to be as quiet as possible, but I am not really fooling anyone about what's going on in there if they don't hear anything? Here is why I decided it would not work. When I put things in a shallow cabinet I tend to want to push them to the back so they don't fall out when I open the door the next time. Well in a double sided cabinet there is no back, you can't lean anything against the back, because it's the front in the opposite bathroom and it will fall out. So you would have to make sure everything was in the center. That's the big reason I passed.

yeah, it's a different order, completely..i'm officially temporarily lost))) in short I meant the pic where narrow white glass sconces are actually installed horizontally over the mirrors, but one can install them vertically too. I think it's currently last pic in your last post? what HKO HKO has is similar in that they're being narrow and of non-transparent glass that'll give out less harsh, more flattering light my second choice would be simple traditional one with fabric..on my screen it's the middle one right now double check sizes and all..you never know, maybe will become a matter of natural selection:)