Ergomake can spin-up previews with applications from multiple repositories. Imagine, for example, that you have a frontend repository which sends requests to a backend repository. In that case, if you want your pull-requests on the frontend repository to include an instance of the backend application, you can reference it on your compose as if it was in an external folder, as shown in the example below. 
# Here's an example docker-compose.yml file that goes in the `frontend` repo
version: "3.8"
services:
    web:
        build: ../frontend
        ports:
            - "8080:8080"

    # You can build a second repository by referencing a folder with
    # the desired repository name in a path *outside* your current repository.
    api:
        build: ../../backend
        ports:
            - "3001:3001"

    database:
        image: mongo
        environment:
            MONGODB_INITDB_ROOT_USERNAME: username
            MONGODB_INITDB_ROOT_PASSWORD: password
After you commit this file to your .ergomake folder, every pull-request in the frontend repository will build the Dockerfile in the backend repository and include it in your preview environment. If you want your back-end repository to include the frontend repository, you’ll have to commit a similar compose file to the backend repository. This time, however, you’ll refer to the frontend repo as an external folder, as shown below 
# Here's an example docker-compose.yml file that goes in the `backend` repo
version: "3.8"
services:
    web:
        build: ../../frontend
        ports:
            - "8080:8080"

    api:
        build: ../backend
        ports:
            - "3001:3001"

    database:
        image: mongo
        environment:
            MONGODB_INITDB_ROOT_USERNAME: username
            MONGODB_INITDB_ROOT_PASSWORD: password
By default, Ergomake will use either branches with the same names or main, if an eponymous branch does not exist.

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Environments with pending branches

When you have pending changes in both repositories, you can ensure that the previews for your branch will use pending changes in both sides. For that, you just need to give branches the same names. Assume you’re doing changes to your front-end on a branch called my-feature. If those changes depend on changes from the back-end which haven’t yet been merged, you can push a branch called my-feature to your back-end repository too. Then, when you open your front-end pull-request, the preview environment will use the back-end changes from my-feature instead of main.