Port-forwarding and website hosting: Two domains, same ip, different computer

So I have two domains: http://firstexample.com http://ift.tt/1GHPyjm

on my router I currently have portforwarding so that when someone goes to http://firstexample.com, they see the website that my computer with internal IP 192.168.0.15 is hosting

How can I set it up so that if someone goes to http://ift.tt/1GHPyjm, they would see the website that my other computer, with internal ip 192.168.0.16 is hosting?

Both of my computers have the same public IP.

Is it a setting on my router I would change? Or would godaddy allow me to configure something?