Sorry, you can't do that due to the OEM EULA.
The OEM license is bounded to a specific PC, and when you upgraded the first Windows 7 PC to Windows 8, you bounded your Windows 8 Pro upgrade license to a specific OEM licensed PC. This means that you can't legally transfer the Windows 8 Pro upgrade license from a PC to another.
But this does not means that you can't successfully activate Windows 8 on the second PC... it will probably fail the online activation, and Windows will redirect you to phone activation. There an operator (or a recorded voice) will ask you on how many PCs this copy of Windows 8 is installed. If you answer "one" they will give you an activation code. This seems a "legal loophole", since the EULA states that you can't transfer licenses, but the activation service seems to allow that... Of course, remember that you have to restore Windows 7 on the first PC (you can't uninstall Windows 8, you have to do a clean install of Windows 7 using the OEM DVD provided with the PC).