The additional passenger feature has been explored before but it comes with numerous complications unfortunately.
What happens when the owner hops off? (dismounts everyone else perhaps?)
How is the positioning of the second player handled? (Server is unaware of the physical positions of a dragons body, making it difficult to position additional passengers)
Dragons DISAPPEAR if a non-owner is mounted and logs off, a vanilla "feature" that hinders this.
Its ok if its a hard task but it would be cool if this worked on 1.20.1