I live in a large flat which has previosly been two separate flats, thus I have two separate incoming high speed internet lines - one in each end of the flat (same price for me whether I use one or both). In one end I have a Time Capsule and in the other an Airport Express. How do I get the maximum out of this situation, i.e. high signal wireless in both ends, but only having 1 common wirelss network setup. As far as I can see, a roaming network would be the way, but that seems to assume you physically link the two basestations - this is not an option here - and also seems stupid with the two separate inlets.
If your goal is to utilize two Internet connections I would recommend that you consider getting a router that can support multiple WAN ports. One example of this kind of router is the Cisco RV320. You can still use your AirPort base station in this scenario in a roaming network configuration with this router.