Hi
A common strategy to employ on Doubles relies on the fact that when you make comparisons of payoffs such as a $2.00 1st Leg winner an a $12 second leg winner this will generally return a higher dividend than a $12 an $2.00 winner.
So the idea is to leave out the favoured runner in the 2nd leg of your bets this also reduces the amount you have outlayed, plus it will give you the advantage to hedge your bets if your 1st leg was successful.