RIght, you need to add switch, but Im not going to give you detail because I don't get paid. This is real trick way to set it up.
i aint get paid either. In order to use two public IP addresses (they are static since you get two of them).
First hook up the dsl or cable modem to a switch box. then hook up first video phone directly to the switch. This vp has first public IP address. A router is used to connect computers, so its hooked directly to the switch. This router has second public IP address on the WAN setting. Hook the second videophone to the router and set the router DMZ (or port forward which ever you like) to match the videophone IP address
Simple! Im networking expert, too.
I helped a client with three public IP addresses with his two videophones and computers and he is very happy. He even dedicated his spare bedroom for that purpose! He got lots of bad advice from others until I helped him. I saw lots of cables, routers, switch boxes everywhere before I helped. less cable snakes on the floor and he returned extra routers and switches to the store.