We don't have great support for orchestrating multiple stacks via a single pull request, but definitely feedback we've heard from other and have plans to investigate.
I opened an issue that you can drop notes into or +1:
https://github.com/pulumi/pulumi-cloud-requests/issues/267
In the meantime, you could consider a slightly different workflow:
• review stacks for BE monorepo are configured to point at a static "dev" stack for the Common infra. PR comments for this stack would just have your API URL, and developers could run the FE locally using that API URL
• review stacks for the FE monorepo are configured to point to a static "dev stack for the BE infra project. If a developer does want to open a PR that depends on API changes, they could change the review stack config manually to point to a BE review stack.