There is not a core command that synchronizes two independent specs/ trees. I would keep app1 and app2 as separate Spec Kit projects, and synchronize the interface contract, not the whole feature spec.

For a fix that crosses both apps:

1. Define the affected API, event, or data contract once. Keep that artifact in the producer repo (or a dedicated contracts repo) and version it.
2. Create a small feature/fix spec in each app. Each spec should link to the same contract version and to the other repo's issue/PR.
3. Run /speckit.plan and /speckit.tasks independently in each repo. The plan workflow already treats contracts/ as the place for interfaces exposed to other systems.
4. Implement in a compatibility-safe order: normally the additive/backward-compatible app2 change first, then the app1 consumer change, and remove the old contract only in a later change.
5. Add a contract or end-to-end test at the boundary so drift fails in CI instead of being discovered manually.

Spec Kit is directory-scoped, so this separation is intentional: each project owns its own .specify/, specs/, constitution, and feature state. The monorepo guide documents that model, and the plan template describes generating interface contracts.

If the main pain is coordinating branches across sibling repos or submodules, the community catalog now includes Multi-Repo Branch Sync and a Multi-Repo Branching preset. Those can coordinate repository mechanics, but I would still avoid duplicating one spec into both apps; a shared, versioned contract plus two local implementation specs gives you clearer ownership and review history.