group membership between two IPS instances

I am trying to setup my users (forum) on one instance and my customers (shop) on another,

I can change members group when a purchase is made on my shop instance; i can’t find a way to synch group membership on the other instance (forum)

Is it possible to synch group membership between two IPS instances when SSO (IPS.Connect) enabled?

Can someone explain the logic on how this work from a technical aspect?

There seem to be a two way connectivity <->

A one direction connectivity -> or <-

Or not enabled; but configured!

The first two option seem to do the same thing,

both <-> and -> <- directions will add or delete the user on both instances simultaneously, is that by design?