I need the formula for laying two horses in the place market, now i know i can't make a profit if they are both placed but if one of them is placed i would like to return a set profit plus the liability lost on the other runner.

For example 2 runners priced @ $1.50 and $1.70

If the $1.50 horse runs a place the other horse will retun me say $10 plus the liability i had to pay out on the plaed horse.