I think what you're looking to do is solo mine with 2 computers from the same bitcoin.exe client, hosted on one computer right?
To do that, set up your bitcoin.exe and guiminer on one computer. In Guiminer set the bitcoin.exe client path on that computer from the "solo utilities." Create the solo username and password (from solo utilities), this will let your other computer connect.
I found this easiest, but on the bitcoin.exe shortcut you use to launch the program, right click and hit properties. in "target" go to the end of the line, add a space, then put
This will make bitcoin.exe start up as a server.
On computer #2, you only need guiminer. Set the server to "solo." For the host, input the ip address of your first computer, if its on a lan, use its 192.168.1.xxx address. Port is 8332, then use the username and password you created on your first computer. Set the device and flags, then hit start mining.
If you cant connect, you might have to go into your router your first computer is on and forward port 8332 to allow connections.