Cisco AAA/Identity/Nac :: ACS 4.2 - IP Pool Allocation Based On NAS Port IP Address

using ACS 4.2 and I can't find a way to bind an incoming NAS port to a specifc IP Pool:
 
When a user connects the request to auth comes from 2 possible NAS ports randomly (this cannot change). Depending on which NAS makes the requests determines the IP range required, so I need 2 IP Pools. There is no way to say 'if request comes from NAS1 give IP from Pool1 and if request comes from NAS2 give IP from Pool2'
 
I have gone around and around with NAFs and NARs, but cannot do this.I can create 2 ACS groups with the specific NAS and specific IP pool within, but then I cannot have a single username bound to both groups.
 
I moved the auth to an AD group in the hope that I could bind that single AD group to the 2 ACS groups; and so have a single username, but no joy.