You may find it easier to use the Mendix DataHub as it allows applications to share common data.
https://docs.mendix.com/data-hub/
There are also Mendix business events that you can use to share data when certain events happen.
https://docs.mendix.com/appstore/modules/business-events/
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.
https://docs.mendix.com/howto/integration/publish-rest-service/
https://docs.mendix.com/refguide/integration/consume-a-rest-service/
https://academy.mendix.com/link/modules/137/lectures/1242/6.1-Exposing-REST-Services
https://academy.mendix.com/link/modules/138/lectures/1226/5.1-Consume-a-REST-Service
I hope this helps.