Re: Double pedalling
Any single stroke exercise for the hands will work great for the feet. Also the first 3 pages of stick control is good to practice.
8 on a hand type of exercises are really good because you can really focus on making the strokes as relaxed as possible.
The best approach I've found is to start at a comfortable tempo for 5-10 minutes and work your self up by increasing by 10bpm and stay there for 5-10 minutes until increasing again until you reach your goal tempo. This way you practice those difficult in between tempos.