As others have indicated even though the Mac Pro PSU has a total capacity of 980 watts not all of that is available for video cards. Again as others have indicated using a standard Mac Pro the maximum you can allocated to a single video card is 75w from the PCIe slot itself, plus 75w from each of the two 6pin PCIe power cables making a total of 225w. (3 x 75w.)
If you had one video card that needs two 6pin power cables and a second video card that needs even a single 6pin power cable then you cannot do this using the standard built-in power supply. Even potentially a single video card might exceed the official limits if it needs an 8pin connection plus a 6pin connection. The Mac Pro does not itself support 8pin connections which can give 150w each.
To drive a second video card that needs additional power you need to add a second power supply. In theory this could be an external power supply but there is also a way to fit a second power supply in the space that would normally hold a second optical drive. See this page and the two links listed on it - http://www.macvidcards.com/do-i-need-an-additional-power-supply.html
Note: An additional power supply could be chosen that does support 8pin connections as well as 6pin connections.
