Vijay,
The Request/Reply scenario its all depends on your source/target requirment.
yes,you can do Pub/Sub and Subscribe request document from source and get response from target SP and Publish it to Source using the same broker ,for this u dont need a different broker unless your source and target connect to same broker.If they are in remote then you need to use Territory/Gateways to connect the local broker and remote broker in sync.
you are in right track,so go a head with Pub/Sub request/response (Synchronous or Async)
Sorry if it is not clear,
HTH,
RMG
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