I have run dual-head under MicroSlop 98 for four years now on three different systems. All three allowed for full dexktop expansion and drag between monitors.
First system used two dissimilar PCI video cards w/8MB of memory each. It was relatively stable except when highly math-intensive apps were expanded to both screens (CAD or big spreadsheets). Of course, these are the kinds of apps where you would want to use multiple monitors. The lockups experienced would freeze everything requiring a reboot (lost work, etc.) Problem seemed to trace to a video memory cache issue.
The next system used a 32MB AGP card and one 16MB PCI card. Same thing happened but not quite as often.
I upgraded the video card to a Matrox Millineum 450 Dual-head card thinking that this would resolve the lockup issues under Winblows. It only "sorta" did. The only lockups and BSOD's that it does now, happen if I strech a 3D AutoCAD screen with multi-view tiled viewports across both monitors. This would be really cool, if it worked, because the different views would (theoretically) update in real time as one made design changes. WRONG......it still crashes despite tweaking every setting known to God and man....but what else should I expect, it's running under Windonts.
All other apps that I use on a job level (PhotoShop, Flash, etc.) seem to handle dual-head world with ease. I also love the convenience of having different apps running on different monitors and being able to drag and drop between them without having to toggle. This is great for tech writing because you maintain vour view of the document being created....real time saver.
I will be watching your thread with great interest to see which applications work well under Linux in a dual-head setup (and with which GUI). This is the kind of info that I will need if and when Linux application development progresses to the point where I can economically migrate my professional software tools to the Linux OS.
Kindest Regards.
Sleeping Dog