It works but you have to keep it at least "semi-active". Some shops have CI services setup to cross compile etc. already. Mainly I've seen this not as a tool to maintain a "backup" but as a way to shake out bugs and keep non-portable stuff from creeping into the codebase.

You could probably do most of it with conservative linting and some in-house knowledge of portability issues, non-standard compiler extensions, etc.

It's typically a lot easier to do this for different compiler, same target, than different targets.