Unfortunately, I think the short answer is no, that’s not possible currently.
A preview environment is generated from a blueprint, and a blueprint is connected from a single repo.
While services defined in the blueprint can be configured to pull from other repos, the preview environment would be created from PRs created against the blueprint’s repo.
Maybe if the non-blueprint repo PR was created first, that service definition in the blueprint could be updated to specify the branch: as part of the PR that contains the blueprint. I’ve not tried this, but it feels possible. However, it’s also pretty manual, and you’d have to remember to revert/remove the branch: when merging that PR.
I’m also struggling to think of how this could be implemented in an automated way - how could it be determined that two separate PRs, from two different repos are related for a desired single environment? Feels like a case for a monorepo.