2.4Ghz and 5Ghz

By "old airport express" are you referring to the 802.11g or 1st generation of the 802.11n model? If the latter, then this Express can only operate on either the 2.4 or 5 GHz band ... not both simultaneously. Therefore the extending Express can only extend one or the other as well.

I would second your recommendation to "switch" the routers using the newer one for your "main" router and the other to extend it.

Note that the 2.4 GHz band has much greater range. Depending on the distance you place the extening Express it may only be able to extend the 2.4 GHz radio.

To get a better idea on how to place your extending base station for maximum performance please check out the following AirPort User Tip.

It is always a good idea to start from "scratch" when setting up a new network configuration. You can still use the same Network Name and password with the new configuration. I would suggest that you start by performing a "factory default" reset on both AirPorts. This will get them back to their original "out-of-the-box" configuration.

In this configuration each base station will broadcast an unsecured wireless network with a Network Name of something like: Apple Network NNNNNN. You can then connect to the main base station, and then, use the AirPort Utility to configure both of them for an "extended" network.