I've been redoing a number of textures, including doors. I've been find single door textures and making them into textures for swinging/sliding doors, by making them symmetrical (see the first two images.)
Both types of doors use old scripts found online, and work by doing terrible things to prims. The swinging door pivots on the middle of the prim, but it's been "cut" in such a way that horizontally half the prim doesn't show. The lift doors are based on an old airlock script - see https://www.outworldz.com/cgi/freescripts.plx?ID=22 - and the prim is contorted in such a way to be flattened, showing three faces on one side, and two face on the other. When touched, the script contorts the prim even further and the middle pulls back, giving the illusion of a Star Trek type sliding door.
The advantage of using textures that symmetrical along a central vertical axis, is that once you've set up one door with script and textures, you can copy it and then drop in textures for either side to make a different door. You could have a plain door one side, and a similar texture + sign on the other. The sliding door's a bit more complicated. One side will show both sides of the door texture, but the other side will dhow reflected versions of only the left hand side of the door texture (see below).
That means you can safely add asymmetrical elements (e.g. signage) to the "front" side, but not the "rear" side. For the rear to work it needs a texture that's symmetrical both ways! That means putting any door handles midway vertically, and they have to look the same up and down! No wonder then this was used for airlocks and lifts!
Of course most free textures you get online a flawed for this. If you do a casual inspection of door textures online you'll find that door handles are seldom in the middle vertically on a door. Usually the door handle's going to be at a height easy to reach with a hand. Hmm, might have a bit of fun with the lifts.
