You need to do this in the Full Mission Builder , or by editing the .mis file.
I recommend you start with the FMB.
Open the FMB , then navigate to the Mission(s) you wish to modify one at a time.
Open the Mission.
The Player Flight can be identified by the little flag attached to its Waypoints.
Move your cursor to any of the Player Flight Waypoints and click on it.
While it is flashing , open the "View" Menu and open the "Objects" Menu.
Your Player flight will show several Tabs :
Properties/Waypoint/Waypoint Options/Plane 1/(and possibly) Plane 2/Plane 3/Plane 4
First things first , you need a Plane for your friend. If you only see Plane 1 , you need to give the flight another plane.
Click on the Properties Tab - one of the Menu Selection is Planes - Click on the Arrow at right whcin will drop down a menu from which you can select 1 , 2 , 3, or 4 planes for this flight.
Once you have selected the number of Planes for your flight , you need to go to each of the Plane Tabs and see which one has been selected as the Player Aircraft.
Disable this by clicking on the Check box beside "Player".
Now when you open the Mission in Coop Mode , you will be able to choose any aircraft in the flight.
Now I recommend you go to EVERY OTHER FLIGHT on the map one at a time , and in the Properties Tab , select the
AI ONLY Checkbox. This will prevent you from seeing and knowing every other flight in the mission , and ensure that you don't select the wrong aircraft when you are in the Coop Selection Screen.
Now you need to make sure this mission is in the Mission/Net/Coop Path , so click on the "File" Tab ,
Click "Save As" , and follow the path Net/Coop/ and name the Mission whatever you want by typing it in the File Name slot and click Save.
Now exit the FMB , and select Multiplay. Select Coop Mission Type , and all available Coop Missions - including the one you just named - should appear in the selection box to the left. Select your preferred Difficulty settings and launch the mission.
When it loads , you should see only the aircraft in your flight available to select.
Good Luck
Kopfdorfer