We need a bit more information from you in order to be able to provide a correct answer to your question.
Which models of AirPort Extreme do you have? If you are not sure, look for the model number on the bottom of the device. It might on a label, or embedded in the foam base depending on the model that you have. The model number starts with an "A" followed by four numbers.
Please post back with that information.
Next, you did not indicate "how" you want the AirPort Extreme devices to connect to each other. Would this be with an Ethernet cable permanently installed between the two Extremes, or do you hope to be able to connect the second Extreme to the first using a wireless connection?
Normally, users configure two AirPort Extremes to provide the same wireless network signal, so devices can move from one area near one Extreme to the other area near the other Extreme and stay on the same network. If you really do want two separate wireless networks, users will have to log off one network when they leave an area near one Extreme and then log back on again to the other wireless network near the other AirPort Extreme.
Is that really what you want to do?
Finally, we don't know what operating system your Mac or PC is using, so we cannot provide the correct setup steps without that information. If you have a Mac, and are not sure what operating system it is using, then click on the Apple icon in the upper left corner of the screen and then click About This Mac. Post back with the OS X Version that you see there.
There will likely be more questions, but that basic information will determine which path to follow.