AFAIK it's all of them, though I have not had occasion to test it. The rocket I used had a 5m to lots of 1.25m and one 2.5m active, and a 5m to five (I think) 1.25m. The reason I think the nodes and/or CoM are off-center is because when sitting on the launch pad, with SAS off, Kerbal Engineer tells me there's a 0.1° thrust offset, when it's always 0° for single rockets. Thanks!