Shall we create a get queue by installing discovery server in Primary hub whereas attach queue in the central hub ?
Can you give us some more information about what you are trying to achieve?
There should be only one discovery_server running at any one time in any one domain.
discovery_agent can be deployed to multiple hubs. Typically this is done when only that secondary hub has access to the robots. When deployed to a secondary hub, it creates a temporary queue. For more info, see:
Discovery Probe Deployment - CA Unified Infrastructure Management - 8.5.1 - CA Technologies Documentation