Retail discs are usually early releases (10.6.0 or 10.6.3) lacking driver support for newer hardware models.
Discs bundled with specific hardware models have full driver support and are usully more recent versions (10.6.5 or 10.6.6) but are also restricted to running on just that model or release series. They also contain install packages for any bundled applications such as iLife or iWork.
In most cases the retail disc should work and you should immediately apply the latest Combo update to add any missing drivers or feature support. If the retail disc gives you any errors you'll need the bundled disc.
Not aware of anything quite like BeTwin, but you may not be aware that is is possible to run 2 user accounts simultaeniously out of the box without the need for any extra software, at least in Lion (And I have seen, but cannot find, that this can be retroconfigured on SL). You mention SLin your question, but have also tagged, Lion, so I will reply anyway.
When you use screen sharing, you are able to choose which display to connect to, either the physical or the virtual display. Connecting to the physical display will allow you to take over the screen of the target computer, taking control of the kb/mouse etc. But if you use the virtual display, you are able to log in as a new user, and run processes entirely independently of the peron who is physically logged onto the machine.
Whilst I have only tested this with a a Mac initiating the screen sharing, there is no reason why with an up to date VNC client any old netbook, or thin client etc shouldn't be able to connect in this way, as the Screen Sharing technology is only VNC under the hood.
Edit: Read this for more information on getting this working using Snow Leopard
Best Answer
Real FullScreen applications will normally run on the Primary Display and they will rarely allow you to configure that. Some applications, however, are running in a “false” full screen, and are just “maximized” applications on top of everything, including the dock and the MenuBar. Some of the latter can be moved by applications like SizeUp and similar (note the usage of the word ‘some’).
In order to change what your primary display is, you have to go to System Preferences -> Display and the click on the “Arrangement” Tab. And then you have to click and drag the white menubar to the display you want to make primary. Take a look at the attached picture. Remember you have to click on the white square representing the Menu Bar and drag it to the other.
The consequences of doing that is that your Dock and your Menu Bar will be moved onto the other Display.