>>>>what steps can be taken care in SAP ECC side , for the proxy to point to two PI integration servers at the same time , PI 7.1. Production and PI 7.4 (newly developed=)
Are you trying to connect ECC production system to PI production and PI development system in parallel? Why do you want to connect a production instance and development instance?
Azhar has already provided pointers and you can find lot of documents on SCN about IDOC_AAE/ Proxy using SOAP.