You can keep both apps with their own IDP and avoid coupling by using a third IDP as a broker (like another Keycloak instance).
This broker handles login via both app1's IDP and app2’s Keycloak using OIDC.

Basically:

• app1 stays as-is

• app2 uses Keycloak

• the broker gives you SSO between both

This way, each app manages its own users/sessions, and the broker keeps a global session across them.

I wrote a guide on how to set up multiple identity providers in Keycloak if you want to go that route:
https://medium.com/@raf.lucca/one-login-many-sources-oidc-sso-with-multiple-identity-providers-keycloak-08cf3cd13c78