they do not have to be the same.
What you do is you map the two message schemas to the one destination.
Steps:
Open up an orchestration
on the orchestration view make sure you have the massage instance for the three schemas.
Drag the transform shape to the orchestration.
double click the center of the transform.
Select New Map radio button
Click Source under Transform.
Select you two source messages under Source Transform: on the right.
Click Destination under Transform.
Select your destination message under Source Transform: on the right.
OK
Then a new map will be created under the project.
You can open it an drag what you need from both the source schemas.
Moja