Icinga to Icinga passive checks

I have some Icinga2 master installations each controlling their own agents.
Now I want to add some external passive monitoring on a standalone external Icinga2 installation which only controls the status of each master Icinga2 installations (plus some external services which is not a problem).

I know that I can switch master to satellites and create a single external master installation which controls every satellites, but I’d prefer not to go this way to keep the configuration simple and have the actual masters independent as they are controlling different infrastructures.

What is the best and simpler way to add a single passive check from an Icinga instance to another just to understand if it is alive and if it’s sending notifications?

Thank you!