Send message between diagrams
The auto-fill feature is based on a naming convention between process variables and message content when the message is throw by a process (i.e. a pool) that is part of the same diagram.
When sender and receiver pools are in separated diagrams the auto-fill cannot be use. You will need to define message content mapping manually. In the same way you need to define the name of the catch message manually (message name will not be listed in the drop down list).
Even if having both pool in the same diagram make association of throw and catch event more easy (as well as data mapping) I recommend to keep them separated in two diagrams to help collaboration (isolation of editors on each diagram).
Thank you very much the answer is cristal clear.