IS connects to a specific Broker hosted within a Broker Server.
The scenario you describe is doable using a Broker territory. Place Brokers A and B in the same territory and they will share document types, permissions, etc. Clients of B can subscribe to any document type. If a client of A publishes a document of that type, the clients of B will get it.
You can also connect territories via territory gateways.
The docs have all the info on setting these up.
#webmethods-Protocol-and-Transport#webMethods#Integration-Server-and-ESB