In a paired namespace scenario, make the primary namespace do the syphoning in a disaster recovery situation.
Currently, in the paired namespace scenario detailed in https://docs.microsoft.com/en-us/azure/service-bus-messaging/service-bus-paired-namespaces, "at least one of the clients" needs to run the syphon.
I don't think it's necessary to have that on the client side. This creates all sorts of requirements on the client side, such as availability.
It's probably much more convenient if a primary queue/topic knows about its secondary queue and starts syphoning on its own as soon as it goes online again.