Simultaneous use of Airport Express and Time Capsule

You just extend the network created by one of the two devices. I.e. you start with the Time Capsule, making it create the WiFi. The AirPort Express then gets configured to just extend this network. If you've setup the Time Capsule to use the 5GHz band then yes, the Express will use that too.

It's effectively just one WiFi network (or two, if you count the 2.4GHz fallback for older/mobile devices) powered by both stations. All devices connected to that network have access to both stations and they will always get connected to the base station which is nearest. Here's this setting in the AirPort Utility:

When setting up a new network with the AirPort Utility you will be asked for this, so it's really easy and happens in two clicks/taps.

Apple also has a good support article on this: Wi-Fi base stations: Setting up and configuring an extended wireless network