As per your scenario what i will recommend is:-
1. You will have to install 8 SMD agents(6- vitual+2 physical)
2. Also install 2 saphostagents on 2 physical nodes.
Hope this answers your query.
For more info:-
http://wiki.scn.sap.com/wiki/display/SMSETUP/Diagnostics+Agent+and+HA+Support
Regards,
Himanshu