"Apart from the feature race, the tipster who posts second in each match, isn't allowed to pick any of the horses chosen by the tipster who posted first."

Seems like an unfair rule. If there is an obvious favorite [or personal choice] etc. it means that if an opponent gets first choice they own it just because they get it up on the board first. Meaning the slower onto the board has to pick a horse for a winner they might not or do not believe will be the winner? What difference does it make if each punter puts up the selection they think will win? Why does the first to put up their selection 'own' that selection?

It's your comp. Sportz and you can run it anyway you like, but perhaps you should give this rule some more thought.