Is there software that allows multiple users to simultaneously and independently share a personal computer running Mac Os?
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