The correct way is to assign a proportion of the $50 outlay,corresponding to the chance you give the horses,regardless of the odds on offer.
If you can't split the two runners,bet $25 on each.
If you fancy the shorter one a bit more,put more on it and less on the rougher one.And vice versa.
But if after deciding how much to invest on each horse,you find that one will not return a profit overall,leave that wager in your pocket.