I can't confirm it by actually connecting two of them to my own Mac Pro; however, you should be good to connect two Apple 30" Cinema Displays as long as you have the dual link DVI adapters. It's probably a good idea to connect each one to a port that's on a separate Thunderbolt Bus as well (the top left port and top right port should work). Since four 27-inch displays with Thunderbolt or DisplayPort Mini are supported, you can probably connect four Apple 30" Cinema Displays with the dual link DVI adapters.
Although, maybe sell your 30-inchers and pick up six Thunderbolt displays? (All _six_ are supported.) Lol.
-Warren