messing with power electronics is really not for the faint of the heart. already got electrocuted by a few kilovolts during a homebrew ESD testing.
I already did modify a single PSU from 12V to 48V. I rewound both the transformer and the inductor. daisychaining would be easier though.
there are also other techniques to increase the power output from a PSU, but you may be burning lots of transistors in the process.
never mount daisychained PSU's in a metal chassis. most of them use the chassis as ground (either Analog or Earth Ground). insulate them if you should. some cheap PSU's also don't feature current limiting (I don't remember if it's specified by ATX Spec.), overload them then the fuse will blow. mid-end to high-end PSU's would happily accept shorts.