This is pretty seamless. If I go by your example it is two routes in the same service. In that case from app1 to app2 you would do a localhost:/app2 from app1.
But say you have two services. Service1 and Service2 each being accessible like https://service1.mydomain.com and https://service2.mydomain.com. and say you have deployed them in the namespace dev. So now if Service 1 wants to connect to Service2 you would simplify call service2 like http://service2 from Service1 and underneath kubernetes would make it a fqdn like service2.dev.svc.cluster.local
Hope that helps