as long as they're not literally touching (which could cause them to rub together and wear) then it's fine. They are better off either being very close or use a cowling between the two. Otherwise air that has passed through the IC will try and escape around the side/bottom of the rad (path of least resistance) than go through it. The same theory applies to putting a cowling in front of them both too, air will try and escape around them than go through them unless a cowling traps the air in at the sides, top and bottom.
