each PCIe slot delivers at most 75 W, so that you get at most 150 W for the 2. By splitting these 2 slots between your 2 cards, each will receive at most 75 W, i.e. 150 W when added with the PCI slot supporting each card. Considering that 5870 cards require 180 W to function properly, you see that they will be under powered. This may work but at the cost of stressing PCIe slots and probably running 5870 in an energy saving mode; i.e. with poor performances. If you wish to run 2x5870 at full speed, you really need an external PSU. In other forums, procedures are described to power on and off such extra PSU when MP is on or off..