So what are they swapping? Band members? Songs? Toothbrushes? Ewww, definitely not! Actually, both bands are recording new albums in the same studio in Franklin, TN. And since between them, they have more gear and equipment (see: guitars, amps, etc.) than we could possibly imagine, they've been nice to each other, swapping gear in and out of the studios.
What would a combo Relient K/The Almost album sound like? Who knows? For now, the guys are just swapping gear. Maybe they'll swap toothbrushes if they ever go on tour together. :) 
Oh, and don't forget to check out this video update of a new song by The Almost (it took them 30 minutes to record he word "hey"!) (and Aaron's voice sounds AMAZING!) (OK, go watch the video!)