I think they split the $$'s because both horses finished second in the race, but each was not the
only second place finisher, so they split the pot. I can't produce the rule, but I'm sure someone here can do so.
If it had been a dead heat for the win, the tracks do it the same way, which is really galling. Imagine if you bet a 20-1 shot who dead heats with a favorite. The chalk player ends up with more than he/she deserves, and you end up with less.
If I'm wrong, I'm sure someone here will correct me.
JK