Sorry, I don't think there is a solution for this. This due the way MWS is programmed, which is somehow related to the way the underlying MaxMSP Framework handles the problem.
Maybe you could get a better framerate with SLI on windows. But I dont have experience with.
On OSX its definitly not possible.
One problem (as far as I understand it) is due to the lack of sharing of textures between the gfx-cards. Since OSX doesnt support SLI there is no way to get any decent framerate over two GFX-cards.
The other problem is that MWServer layzily has only one outputwindow that it spans over the output-diplays. if the output-displays are on the same card (like the desktop) than that is no problem, but as soon as one of them is on a different card, the output-texture has to be shared by the two gfx-cards, and since the gfx-cards cant share textures directly, it has to be done through the PCI-bottleneck - hence the low framerate.
(this is what I think I understand of this problem, this understanding was gained out of dozens of hours of reassembling gfx cards and programming tests and staring at low framerates and frowning at stuttering video-playback)
1. SLI on window might help, but I dont have experience with it. Would be interesting to hear from somebody that has got some.
2. Use ONE powerfull GFX card and split with Matrox TrippleHeads. The new MacBook Retina works sweet with two Tripplehaeds attached. MWS can run 6 projectors this way with a resolution of 1280 x 720 each with a solid 60 fps. It might drop a bit with a Syphon stream and another Videoapp running, but even with a syphon stream dimension of 5000 x 700 it stayed above 30 fps.
3. Wait and see what the new Apple MacPro will bring. Most likely the GFX-cards work together and share textures, so it might be a very powerfull videoserver. But probably not very affordable.