Re: Cannot switch to another session with 100.14.11.
There may have been a misunderstanding. You can still run more than one X server on a given GPU and switch between them, but access to the GPU is limited to one such X server at a time; i.e. only one of the X servers can be VT-switched in (in that sense, no two X servers can "run" concurrently). If this no longer works, it is most likely due to a driver bug. Also, even if it weren't supported, the fact that it appears to hang choroba's system would still be considered a bug if the driver were responsible.
All that said, I'm not aware of problems with VT-switching between X servers; if you experience problems, please provide as much information as possible along with the nvidia-bug-report.log file (i.e. are you switching between bare X servers or do you have 3D applications running on either or both of them, etc.).