Hi James,
I would request you to go through SAP library topic about this type of configuration and one discussion topic very similar to your case. Please go through both of them and then try to configure. If it does not work, we will check again. Thanks.
SAP Web Dispatcher for Multiple Systems - SAP Web Dispatcher - SAP Library
One Web Dispatcher, Multi Systems with URL Prefixes
Let me know if it helps.
Regards,
Sujit Kumar Banerjee.