harddrive123:
Two different computers both connect to one board via USB Vcc, and ground.Because both are ~5V and your sharing a common ground there, any slight differences in voltage will cause a slightly greater strain on one of the computers regulator when it gets pulled up to the other.
Combined with the fact that this happens all the time when you connect any cable from one computer to another that share a Vcc/ground.
Any reason why this wouldn't be true?
Never connect two voltage regulators in parallel, this can lead to instability and
turn the combination into a high power oscillator (worst case). Each regulator
has gain and uses feedback, the combination may have too much gain at some
frequencies - the resultant oscillation can put the full input voltage on the output
rail.
USB hosts do have quite a lot of extra protection facilities so this particular
failure mode may not happen, but I don't expect it to work.