SAP WEBDISPATCHER route the InA services to the same client as its frontend's client on backend.

I have 2 system which are frontend (FRD 300) and backend (CLD), but the backend system has 2 clients (300 and 320).It seems like the SAP Webdispatcher always route the InA service to the same client as the frontend system client. How do i route the /sap/bw/na services to the specific client 320?