Airport express connects to other airport express instead of connecting to Extreme

I set up a network with :

An internet router

1 airport extreme

2 airport expresses

I used the "extend a wireless network" option to configure both expressess (with exact the same config settings)

For a while this structure was shown in the Airport Utility app

=> Airport Express 1 (house 1)

Internet => Airport Extreme

=> Airport Express 2 (house 2)

This worked fine, also with playing music.

After pulling out the Express 2 en plugging it in again, the structure automatically changed and this strucure is displayed:

Internet => Airport Extreme => Airport Express 1 => Airport Express 2

With this last structure all wireless devices automatically connect to express 1 and thats not the desired situation.

What do I have to correct to achieve the parallel situation (structure 1), instead of the wrong serial situation?

Thanks for any help