Hi Siriram,
First of all, the Priyanka idea is for me the best way to achieve your goal, because you have simplicity although may be doesn't fullfil all your requeriments.
Right now, you have two interfaces and you need to split your message in two, this theme is taking in this part of the SAP documentation http://help.sap.com/saphelp_nw73ehp1/helpdata/en/18/fa87311cea41e49dc9d931a3b67703/content.htm. Therefore, you have the interface split or the message-based split option. If you have differences in the data to be send i would try the second option, you have here an example http://www.sdn.sap.com/irj/scn/go/portal/prtroot/docs/library/uuid/90dcc6f4-0829-2d10-b0b2-c892473f1571?overridelayout=t…. However, on six months you will use the proxy for the second scenario and the interface will be the same and Priyankaa idea will the best way. Depending of your urgency, you can go right now with multi-mapping or to prepare two scenarios, one for now and the other when the ECC uppgrade will be done.
Regrds.