More info: What cards have you tried. and how's your Xorg config file?
A couple suggestions, any card using an S3, nVidia, or ATI should work. S3 as supported multi-head since at least as far back as the ViRGE/DX(I have two of those as secondaries) and nVidia should have support for it in anything TNT based or newer at least(My Primary card a GeForce2 Pro). Anyhow, chances are, just about any card you tried will work, your Xorg just has to be set up right.
Anyhow, if you want your screens to all act as one then nVidia(and ATI, I'm sure) has cards that can handle 2 monitors and have hardware screen 'meshing' support so you aren't using precious CPU power to combine the screens... I believe these cards start at $90 though for a GeForce 3 based card...
Here's a place to start your search. I got that by searching 'Dual Monitors in X' at Google. Searching 'Dual-Head Video Cards' should yield some good results too...
Anyhow, peace-out, and good luck, and if you need help or want to run your Xorg.conf/hardware list by me, I monitor all threads that I post to, so just post back here.
Personally I let KDE run a different desktop on each monitor. I can move my mouse between them and control them fine, but when I maximize a window it maximizes to fill the screen it's on, not all the screens...
