Q: can you connect two airport extremes?

Yes. If your goal is to extend the wireless range of one with the other then check out this Apple Support article on how to configure them for a roaming network.

I have two airport extremes.  One the new tower version, just purchased and an older one, several years old the pancake look.  I just installed the new one and would like to use the older model to extend my network.  Can this be done.  If so how.  When I put the older one in proximity of the new model it is not seen on the network even though one of the apple guides said it should be able to be seen.  I want to do this wireless.  The older one used to be my main router.

I am using a PC, Windows 8.1, 64-bit.

Thank you,

Mike

Please start a new post for this, as the original question is about connecting AirPorts using a wired Ethernet connection, and you are asking about wireless.

Thanks.

In the meantime, you might want to review the System Requirements to configure the new AirPort Extreme:

Setup and Administration

• iPhone, iPad, or iPod touch with iOS 5 or later and AirPort Utility 1.3
• Mac with OS X Lion v10.7.5 or later and AirPort Utility 6.3

http://www.apple.com/airport-extreme/specs/

If you are saying that you used Windows to set up the new Extreme, there may be some features and functions that are not operational. Unfortunately, I don't have a PC here to test to confirm.

Bob - I apologize for the incorrect post. 

God Bless,

Mike