Is it safe?
Haha, no. Not because of the technique used but because multi PSU systems that aren't designed for it are inherently prone to more problems. The most common problem is diminished regulation feedback which increases voltage ripple. If the PSU's are of good quality, they should be able to still keep it within tolerable levels but you can't really know until you try it.
Also, there is an even more minimal option:
Hook up all the power cables as normal
Jam a paperclip in the secondary PSU's ATX connector so that it connects the green wire to a black one
Start the computer
