Is there any separate mesh defined for the coupling?

Do I have to provide the interface mesh as a separate file?
Are multiple meshes for each participant required?

No separate or external definitions are required.

The interface meshes are named in the preCICE configuration file. The adapters are then responsible for specifying nodes and optional connectivity information on their respective side of the interface.

preCICE is designed to handle distributed meshes of participants running in parallel.