That's Jake's apartment. Pirta is moving in with him, not the other way around.
And it's actually fairly casual- just as Swampy tended to already spend a lot of time over at Sandy's, Pirta already tended to spend a lot of time at Jake's, and so (in both cases, really) already had some of their stuff swapped over. Toiletries, extra clothes, etc.
The "big move" was really just Swamps moving the last of his stuff out of Jake's, and Pirta officially moving out of her apartment.