Odds on implies favourite
Favourite does not imply odds on.
Except in [mainly] one on one contests like boxing or football. Or in the land of Flatus where apparently arithmetic works differently from the rest of the world.
Bookmaker odds are not probability.
In probability, the odds add up to one, in bookmaker terms, they add up to (one plus tax plus profit).
So, for a bookmaker, there's no problem in having two runners both at odds on