To calculate a Double Bet assume the stake is £10 across the bet. Bet 1 is a 3/1 Single Bet. A win equals £40 return (£3 return for every 1 pound staked plus the stake returned).
Bet 2 is a 2/1 Single Bet using the return of bet 1 as the stake. A win equals a £120 return (£2 return for every £1 staked = £80 plus the original £40 stake)