There is nothing to configure for the CM55 project. Just build it using

Project > Make

on the main menu after you have already build the secure CM33 and non-secure CM33 projects, and this will generate a single combined image at

..\build\app_combined.hex

.

After all three projects have been built at least once, any time you build any of the projects, the combined hex file will be regenerated.

Refer to

Debug configuration

multi-core programming and debugging instructions.