We use Forests and they are pretty awful. I would recommend against using that extension. It is awkward to work with, makes it hard to rollback your repos to a consistent state, and is painful when you have a change that applies to both repos.
Although, actually, I'm pretty sure you can get git projects with submodules to roll back to consistent states. But it might take ten separate commands, five attempts, and a visit to the FAQ. Submodules are conceptually tricky and their workflow is a mess.