You can't have two portals on the same subnet and you also can't have different rules for different captive portal users.
Sounds like maybe what you really want is 802.1x on your switches, which could maybe drop users in a different VLAN/subnet based on their authentication.
Or instead of a portal, block all outbound web access, setup squid + authentication and maybe you can filter by user/group there (plenty of threads about that already)
By far the easiest option would be 802.1x on the switches if your switches are capable.