For some reason, after installing and using the Brink dedicated server through Steam, every time I attempt to launch Brink through Steam it instead launches the dedicated server and wont allow me to launch the Brink game application. Instead I get the error "Brink already running... use win_allowmultipleinstances to force" bu I cant figure out where to apply this CVAR even if it is still functional and not a byproduct of older ID Tech 4 titles.

Any ideas on how to fix this?

Do what it says on the cmdline to your server and your client add:-
+set win_allowmultipleinstances 1