The YJ cast has been running a covert operation keeping an eye on TR for the past few months, so this could be a mutually beneficial partnership for the two teams to trade notes and all that, but I think they'd rather stay allies than officially become part of the group just because they'll want to continue operating as a team with their own agenda (and they've got a few trust issues). I assume their ultimate goal is the same (taking down TR), so working together only makes sense, it'll just be kind of a weak alliance until the trust issues are solved.

Also, Wally specifically will be concerned that having too many moles inside TR will draw attention to Conner, and he will have serious issues about amateurs people potentially blowing Conner's cover. He just doesn't want his other bro (or anyone else for that matter) to end up beaten and broken on the doorstep of a Pokemon Center, and because this happened so recently to Robin, the stakes for being a mole will probably be much higher since TR is now aware that there could possibly be more spies in their ranks.