I have a coupling I need to remove from a 1/2" stub.

The joint on the other end of the stub is only 2-3" away.

If I wrap that other joint with a wet towel, will I successfully be able to desolder the coupling and be able to reuse that end with a new coupling?

I always have bad luck with desoldering and usually have to cut again. In this case, recutting means redoing a significant portion.

Whats the best technique for desoldering in general?
Whats the best technique for soldering directly next to other joints without disturbing them?

your question requires a two part answer.
unsoldering a coupling depends greatly on the type of solder that was used.
the old 50/50 solder or the newer 95/5 type.
50/50 comes apart so much easier.
wraping a wet rag around a neighboring joint is always a good idea for either type solder.
anyway, i find a lower heat for 95/5 works better. slow but sure because it will quickly go from the right temperature to being overheated. once it is overheated, forgetaboutit.
patience...
lastly, it must be absolutely dry to work best. a shop vac helps to suck out any water laying in the pipe.

You can also clamp a pair of vice grips by the second fitting to help act as a heat sink for the heat traveling down the pipe.

come to think of it, perhaps that's part of my problem with desoldering. Say im holding the torch in one hand and grasping the fitting I want to remove with a pair of pliers with the other hand getting ready to pull it apart.

Are the pliers in that case acting as a heat sink?

Yes they are. Heat it and only grab it with the pliers as you are removing it or the metal of the pliers suck the heat out of the fitting.