Have two time capsules and use the one only as HDD and the other as router I have two Time Capsules and I want to use both simultaneously. One as a router and the other as a HDD only. 

How can I do that?
Open the AirPort Utility and pick the AirPort accordingly:

HDD:


Select the Wireless tab
Network Mode: Off
Select the Disks tab
Tick: Enable file sharing


Router:


Select the Wireless tab
Network Mode: Create/Extend a wireless network
Select the Disks tab
Untick: Enable file sharing