Hi,
As of now we do not see such direct option that alerts when one app slows down due to other app in ui, hope that clarifies, request you to define health rule on target tier/BT/app in controller UI under "alerts & respond" section with email alert for now, Let us know if that answers your query.
One other option that comes to me, is retrieve target metric from app1/BT1/tier1 in controller UI using rest api /controler/rest/api (you can copy rest api from analyze -> metrics browser) using custom script part of machine agent and report as custom metric to app2 and now define health rule on app2 on custom metrics such that when value of custom metric is corssing a threshold the heatlh rule/policy with health rule and email alert will trigger.