To have two version of Debian a stable and a bleeding edge one, in a partition

How is the workaround for having two version of Debian firmly installed, a stable one and a bleeding edge one, in a single partition?

How to succesfully have such with the stable being a main/primary and the latest one is just its minority (so sort of a miniature OS inside another) ?

Then would be sort of: It needs some additional in CLI on launching an app that requires the latest Debian as if it gets into that environment, otherwise would mean launch normally app for stable one environment