In the history of the Olympics, has anyone ever competed two horses at same Olympics?
Steffen is on the team with Ravel, and Legolas is looking like he will be the back up. If that is the case and one of the other team members is unable to compete, then Steffen would also ride Legolas at the Olympics.
Has there ever been a rider who has had the pleasure of competing two horses at the Olympics? If so, whom?
No, Steffen cannot ride two different horses in the same competition at the Olympics. I would wager a guess that they will send at least 4 riders and 5 horses, so that they have a true backup combination if something should happen to a human athlete.
Legolas is a back-up ride ONLY for Steffen. There is no catch-riding allowed at the Olympics, so no other rider can ride Legolas if their mount is hurt. The USEF wants to guarantee that their best rider will get to ride on the team, hence bringing a reserve horse just for him. If they bring a reserve TEAM (like Todd Flettrich and Otto) then that team may compete in place of another, but one of the team riders cannot ride Otto if their horse is hurt, etc.
Not in the same event, that I am aware, but Fritz Theidemann competed both in dressage and show jumping at the same Olympics, and Mark Todd competed in eventing and show jumping at one (or two?) olympics. There is a story that people saw Theidemann schooling his horse and concluded that while the horse was "pretty good" it wasn't a threat to the top horses....but the horse he was schooling was his show jumper