Multisample with two monitors

My application must run in two monitors using an unique grapchic card, in my case a nVidia 7800 GTX. I want to use multisample antialiasing in both monitors, then when I create the windows I reserve four samples for each window. The problem is that when I begin rendering the scene a OpenGl error happens telling that there are memory problems. If I reduce the number of samples to two samples, I have not memory problems, but the application runs extremely slow. Besides If I run the application in a unique monitor, with multisample activated i have not any problem, neither memory problem nor speed problem.
I would thank if samebody could tell me how I can render in two monitor with the multisample activated.

Reduce the overall resolution.
Be sure to be in nView “span” mode, with 3d acceleration.
How much video memory do you have ?

According to :
The hardware viewport maximum is (independently from video ram) :
Max. viewport size 4096 x 4096