Good question.

If both horses are "overs" one can bet on both using an appropriate staking plan.

I now use different betting approaches to this problem.

If my main system selects a horse, i will only bet if its paying more than $4, excluding quality group 1 races, where horses like Northerly, Lonhro etc are involved. Or a horse has a superior speed rating for sprints.

For my new roughies method, I place bets independently from my other method.

So i may have 2 or 3 bets on a race.
1 main selection only
1-2 roughies - smaller stake.

Total stake will be such that regardless of which horse wins/places - I will make a good profit.