By multiseat, do you mean allowing two people to use your Linux PC at the same time, using a separate monitor and keyboard/mouse, as of they’re on a separate computer? You can do this without installing additional software, though you must configure the seat from command line:
Wayland seems to support multiseat but I can’t seem to find any documentation or tutorial save for an article on phoronix: www.phoronix.com/news/MTM4MzA
Is it possible to isolate applications per user?
Each user with have their own login session, so ther application processes should be separate from each other.