You have to use;
#showtooltip Ambush
/Cast Pick Pocket
/stopcasting
/Cast Ambush
And you have to make sure you stop moving just before pressing or the Pick Pocket will fail.
The reason for the /stopcasting is to prevent your Ambush or whatever other ability you are casting from clipping over the Pick Pocket.
I still have to loot twice on the odd occasion, but 90% of the time the above does the trick, providing you are using the auto loot addon and have auto looting enabled.