You may find it easier to use the Mendix DataHub as it allows applications to share common data.
There are also Mendix business events that you can use to share data when certain events happen.
If you want to use REST services, then one app will need to publish a REST service, and the other will need to call this. There is a lot of documentation on how to do this, as well as learning paths in the Mendix Academy.
I hope this helps.