Dual cinema display problem and Mac Pro

I have a new 2010 2.4GHZ 8 Core Westmere with ATI 5770. I have always used a second monitor as a pallette/finder monitor. On past G5s and G4s I have always used 2 video cards to lessen the load from the main monitor for Open GL 3D modeling apps for instance.
However with this new machine I thought the 5770 could deal with 2 monitors ( 30 inch Cinema Display as main and 20 inch Cinema Display as secondary ). The ATI 5770 has one DVI and two MDP plugs.
Here is my problem. When I plug the 30 inch CD into the DVI plug and the 20 inch into the MDP via an apple DVI to MDP adapter, it defaults the "main screen" to the 20 inch display.
How can I get the 30 inch monitor to be the primary screen? I've tried switching the cables but this elicits some wonky behavior as well. Do I need a second video card?

Go into the Display Preferences, and drag the little menu bar at the top of one of the displays to the other icon, it should make that one primary

Humbled. Praise to tvick47!!!

Thanks tvick47. I think I may have come across this but had obviously forgotten. Thanks for your solution. It worked as stated.