no, you cant combine them. putting it simply, if you have four power hungry devices (or groups of devices) in your system using less than the 18A, split evenly over the rails its all good. put two of them on the same rail (say, a high powered CPU and a video card or two drawing from the PCI-E slot) and suddenly that one rail might have nowhere near enough power to do its job - while a single (or more powerful dual) rail unit would have no problem. multiple rail PSU's arent bad - its when the amperage is so low, that its a bad thing. these 18A rail units just cant handle modern power hungry hardware.