I've heard it is also important to connect the ground wires on the PSU:s, so they have the same ground level. Is that true? Do you need to do it even if you run both PSU:s off the same power bar?
Once you connect the second power supply to a card that is connected to the PCI-e bus of the motherboard the 2 PSU's are on the same ground level. However, it is better to directly ground the PSU's together because the ground pins of the PCI-e bus are only rated for so much current.