We need a bit more information from you to be able to provide an accurate answer to your question.
Which version of AirPort Utility are you using?
If you are not sure, open up AirPort Utility, click the AirPort Utility menu item in the upper left corner of the screen, then click About AirPort Utility and post back with the version number that you see there.
When you say that the AirPort Extreme and AirPort Express worked in tandem, we don't know whether you connected the AirPort Express to the AirPort Extreme using a wired Ethernet connection....or....whether the AirPort Express connected wirelessly.
Can you clarify on that?