SSO provider logout by another Discourse instance

Say we have forum A and forum B, where forum A is SSO provider for forum B, handling all login logic.

If a user logs out on forum B, how do we log them out on forum A as well ?

Maybe I am overlooking something but it seems like this is not covered without a custom API call ?
There is no logout webhook either, right?

Why the A-B pairing? Different languages? I think it’d be helpful to have context.

It is a situation similar to / inspired by How to manage the buying and selling category?