Gmod client and dedicated server on the same machine

As the title says, i want to host a dedicated server, and join the game on the same machine. I don’t have a proper second PC to host it.

Why? Quad core. Source dedicated server uses all 4 of the cores on a quad core processor, which means better performance (regarding physics mostly). If you host a listen server, garry’s mod only uses 1 core. And that doesnt go well on spacebuild maps in particulair while playing with other players.

I’ve read on forums it’s possible, but they never tell HOW. I googled like hell but couldnt find any useful information.

So, is this possible, and if yes… How exactly?

Thanks in advance for replies.