First, you obviously need 2 configurations for your computers. Lets assume that you have the LPT element configured for the first computer (LPT1). Staying on the same computer, you create a new configuration and add a configuration record that replaces the LPT element with its copy. It should look like this:
Next, you switch to the 'Second computer' configuration and select the new LPT element there. You cannot configure it for LPT3 via a list of detected ports because LPT3 exists only on the second computer. However, you can enter the port base address manually, given that you know the base address of the LPT3 on the second computer. If you don't know it, run EventIDE there, select LPT3 and check the 'Base Address' value.
Once you manually enter the address for LPT3, save your experiment and switch back to the base configuration. When you bring the experiment on the second computer, just switch to the 'Second computer' configuration with the LPT element configured for LPT3.