Unfortunately the correct answer doesn't solve your problem.

Currently X.org is not able to split graphics to two different video cards. A long time ago there used to be a way to do this using Xinerama, however only the proprietary drivers still support Xinerama. IOW, you can't do this if one of the GPUs is Intel Graphics.

Now, you can get this to work if you use two NVIDIA cards using -nvidia and Xinerama (or perhaps TwinView). And maybe two AMD cards using -fglrx.

However, my recommendation would be to buy a new NVIDIA card that has two outputs on it, and connect your two monitors to that. Then you can run either -nvidia or -nouveau and it should give good performance and be stable.

Of course, it would be nice if the configuration you're attempting worked. There are some efforts upstream to rearchitect X to allow it, but this is still in the development stages and may be a year or two. But if you are looking for something to spend some time on as a hobby and aren't afraid of breaking your system, you could experiment with the upstream codebases and supply them with feedback.