I have just recently tried the jockey mode and it's great, opens up a whole new experience within the game. It is very simple to have a go at, although I'm sure there is more finesse to it than I have managed so far (only tried the flat).
The issue I have is that it doesn't seem possible to ride in a race if you have more than one horse entered. Is this correct or am I missing something obvious. It's a shame if that is the case as I almost always have multiple entries in the really big races.

You Can choose which one to ride just by clicking take the reins on one of your horses in the jockeys Orders Scroll through your horses on the Jockey Photo Icon.