Even useless eye-candy in compiz works fine (though it's a bit slowish with RDP over interwebs). Both systems run all the time (well, the Windows needs to be booted occasionally where as KSplice deals with the Ubuntu pretty well), and they work just fine. From work-flow point of view it's mostly like working with a single system.
I also have a console switch so I can access a separate Mac (for OSX) from the same console. It works too, but it's such a pain to hop from one system to another that I usually try to avoid having to bother. What this "hot switching" sounds like is basically like using such a console switch. Doesn't sound so great when you're used to being able to just focus another window.
But it can be mitigated by using external USB drives and the 'dd' command, which allows an entire file system to be stored as an image file and then restored or even mounted temporarily.
No need to use 'dd' as you can just take a tar-ball of one filesystem. That way you don't waste space on storing (and more importantly moving) the garbage in unallocated space. There's nothing special about any of it, except whatever the bootloader (ie. grub hopefully) requires. You can use some Live ISO (usually the same one you used to copy the data over) to chroot into the system once the data is copied over and tell grub to reinstall itself (update-grub). The only other tweak required is patching new UUIDs to
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