Well, the general idea is that if you are logged into one of the apps, then you probably have an Auth0 session established. So when you are trying to log into another app, try to do that w/o a prompt (not asking for user credentials). If you manage to achieve that, then you have a session already.
Not sure why you need that, as usually it’s the opposite: log into app B, if you are already logged into app A.
I think also, that you may be able to use Auth0 actions to achieve something similar, but I haven’t tried to do that personally.