Dante, it is two separate bets at one price.
say you want to bet Man Utd. -1/2 & -1 / +110 over Liverpool.
If your bet is $100, it breaks down like this.
You have $50 at Man U -1/2 / +110
and you have $50 at Man U -1 / +110.
If they win by more than 1, you win both.
If they win by 1, you win the -1/2 for $55, and you push the -1.
If they draw or lose, you lose 100.
Hope that clears it up.
It was confusing to me at first, too.