There are a few older single rail PSU's, Im thinking Corsair units here that have plenty of power but only 2 Pcie Cables.
If you have a highend top quality PSU with plenty of power and enough spare Molex connectors then there is no reason not to as long as you know what you are doing. It is really not recomended and a change of PSU is the correct way to go
. however should you choose to try it make sure you use 1 PCIE on each card and 1 Molex converted cable on each card to spread the load evenly.