Is it safe to combine power supplies?

[darknezz19]

Some have said yes but just as many have said no. One side said the voltage differences that can occur between the separate rails can cause voltage spikes. The other people say the differences are within 12v tolerances and it's not an issue. Also one person said to tie the ground between the power supplies and keep this connection thick and short. I'm hesitant because it would be a pity to loose any amount of hardware  by an instability or whatever. What do you people here think or have experience with? Thanks for your time.

[FireBTC]

It definitely isn't the best idea in the world, but you will probably be fine.