I would not experiment, if you can avoid that kind of stuff just go with school examples.
It should work because two PSU should be on same voltage potential but you never know what could happen if one PSU fails.
Not voltage potential, its called same power phase, if not then everything burns.
Power phase from a Direct Current power supply huh? The inherent traits of a AC-DC switching power supply is that the output is directly isolated from the input. There is no such thing as phase on the DC side of a power supply.
The reason they dont want you to mix power supply feeds that are powering different GPU banks (A, B, C) is so you dont have bank A trying to draw off of B power supply which can lead to excessive current amounts flowing in conductive traces of the motherboard. Amounts of current that are actually higher than what the board traces are rated for. Get in that situation and your traces on the motherboard will become a fusible link and burn open.