Do do this ensure the WHR is on the 1.82 firmware which can be downloaded from the Buffalo website.
Move the switch on the WHR to OFF.
Set the IP of your PC to 192.168.11.99
Logon to the web page for the WHR on 192.168.11.100 => root / no password.
Click WIreless Config, set the mode to Slave (EC)
Click Search and search for the available SSID for the WZR
Enter the same Encryption Type and Password as the WZR => Apply.
Under Wireless Config click Basic 11 n/g/b
Set up the same SSID, and encryption as the WZR.
Put your PC IP back to Automatic once done.
Now you should only see one wireless network but it will cover a larger area.