Instructions to parallel install two Resolve versions on Windows
can be found here.
Note though that Resolve 14 and 15 use different disk database formats, so I recommend either setting up two databases using PostgreSQL, or exclusively using the Postgres database with one version, and only using the disk database with the other.
All of this is unsupported by BMD, so don't pester their support with it. This is very much at your own peril, do not use with production/deadline projects, yadda yadda.