use two time capsules together
I do this by using AirPort utility to turn off WiFi on the second Time Capsule. Configure it for:


turn off the network which places it in bridge mode
allow setup over WAN if you want to plug the WAN port of the second AirPort into one LAN port of the Time Capsule that runs as a router.


Leave all other settings default (so wipe restore the settings) and then give it a name and make the changes above.

So ethernet goes from the cable modem to the first TC WAN port. Then ethernet goes from the LAN port of first TC to the WAN or LAN port on the second TC.