Platform.sh supports building multiple applications per project (for example, RESTful web services with a front-end, or a main website and a blog).
All of the applications share a common configuration through the files present in the .platform/ folder at the root of the Git repository. Every application exists in its own subdirectory, and each of them contain its own application configuration via a .platform.app.yaml file, so that your directory structure looks something like this:
$ ls -a
.git/
.platform/
routes.yaml
services.yaml
app/
.platform.app.yaml
pom.xml
[...]
app2/
.platform.app.yaml
pom.xml
[...]
pom.xml
When you push via Git, Platform.sh will build each application separately. Only the application(s) that have been modified will be rebuilt.