VPN Issue

Hey guys.
I need to set up a VPN connection between two routers. The thing is that both sides use the same range, so they asked me to route the machines through a public IP address.
My questions is how to do this because the vpn device has already an ip address (public), after it makes the tunnel with the other, how can it route the workstations through another public IP???
Hope anyone help me with this or give some URL where I can find an explanation.
Thanks in advance