Multiplayer PC to PC

[LunaMoo]

Internet connection is obsolete for adhoc, but those pc's have to communicate somehow aka be connected in any form of LAN, ie cable, wifi whatever you like.

If you already have that, it's extremely easy, doesn't need any detailed guide(that's probably why it doesn't have one:]), nor any third party software and has biggest compatibility at the same time.
Just go to networking settings in ppsspp, enable the built-in pro adhoc server on one of your pc's and use that pc lan ipv4 as a server address for both ppsspp clients, also make sure you enabled networking in same place and that your mac address is different in both ppsspp(unless you copied ini files it will be, or you have some extreme luck as the chance for it repeating is smaller than winning lottery few times in a row;p).

If you don't have a lan, you will have to get something to connect those pc's together no way around that, there are lots of tutorials in the net how to create lan and lots of ways to do it, so you shouldn't have much problem finding something within your reach.

Other than that, just have to remember that not all games have a functional adhoc on ppsspp, so adhoc not working doesn't have to mean you're doing something wrong.