[MpG]

What I mean is that when the air goes through one radiator, it warms up (because it's carrying away the heat from the water). If the warm air goes through a second radiator afterwards, it won't be as effective. In fact, if both radiators are on the same loop, the radiator getting the warmed-up air often does absolute squat, especially with lower-speed fans. So if that's how the radiators are going to be positioned in your case, than I can't see a lot of point in adding the single-rad to the main loop, that's all.