There is actually some flexibility to how you set these up. Here's a picture of my cubicle:
The two 17" monitors on the right are mounted on a Tyke Supply dual vertical mount.
The other 4 are all mounted on the Ergotron LX Desk Mount. The 2209 on the left in landscape and the lefthand U2311H are each on their own mount.
The two on the right are each on their own arm, but both
are attached to one post.
On the center monitor I added an[ame="http://www.amazon.com/Ergotron-45-244-026-Extension/dp/B00358RIR2/ref=pd_sim_e_1
"] Ergotron extension [/ame]so I can really move it around.
I don't think the mount is rated to hold this much, especially with the extension on one of them, but it is very solid. Once you have the tensions set, you can move them around with one finger.
So to address your dual mount question, you can make your own.