force the user to use a unique NAS

I have two load-balanced NAS with diferents networks, the users can use the first or second NAS to gain network access from dynamic ips via rlm_ipool (two diferents ip pools) radius module. But now, I need to use unique fix ip from some users. How can I force the user to use a unique NAS ? If don´t make this, the fixed ip can be out of network. Make sense? Thx Toledo