Each wing member can share ONE mission at a time with the other wing members. So, in a wing of two, both members can share a mission for a total of two shared missions. You can, of course, change which mission you are sharing at any time - for example because you’ve completed and/or handed in the first one you shared.
When you hand in a shared mission, the other wing members get a message indicating they “helped” which allows them to claim a reward. DO NOT delete that message or you will no longer be able to claim the reward.