They do have a different internal IP in your network. But the ISP sees the IP-address of your router for both PC's.
Try it for yourself:
1. You can see the internal IP with ipconfig.
2. You can see the external IP via http://whatismyipaddress.com/
I have a PC, a printer and a laptop that are in a network. I have one ISP.
My question is 2 computers are online, do they have 2 different or same IP?