This is a follow up to the question "Two macs, one ip address" originally posted by ctaranto. Like the original OP I wanted to reserve a single IP for 2 different MAC addresses (one MAC at a time).
My use case was to have a single DHCP assigned IP to my XBox wired and wireless MAC addresses. Then the necessary XBox port forwards are made to that IP and don't change when I switch between wired and wireless. This makes it convenient when I occasionally move the XBox from it's normal Ethernet connection in the lounge room to another room with wireless only.
Anyway it can be done with Untangle. First remove any existing static DHCP entry for the IP. Then, under Config / Network / Advanced / DNS & DHCP add a line like this:
dhcp-host=<MAC1>,<MAC2>,<IP>
eg.
dhcp-host=58:82:a8:00:f5:d2,58:82:a8:00:f5:d4,192.168.1.100
This scenario may be a corner case but hopefully the solution makes it convenient for someone.