Why should I buy more than one base station?

I have a Linksys Wireless router and just got two of these WN3000RPv3 extenders to put one downstairs and one upstairs from the AP. When I use them on different SSID's it works fine. When I set 1 to the same SSID as the AP it works fine. Once I set the second one to the same SSID both extenders freak out and I cant even get to their management web page. I am thinking what is happening is that the two extenders are connecting to each other instead of the AP but I'm not sure. Has anyone tried this? Is it possible to do?

Thanks in advance