There are a LOT of maps that you can practice double stairs on that don't focus on that kind of pattern. Anything with bracket based patterns will be a big help. Anything with layered streams will help. Doorknob and Davteezy's BMS converts are FULL of maps that would help practice double stairs. The difficulty in playing double stairs is that they are a bracket based pattern.
Consider a double stair starting on the left and moving up: 13/24/35/46/57
The 13/24/35 has partial brackets in it. if you look at a simple bracket such as 13/2/13, you'll notice that you alternate between fingers. If you look at the 13/24/35 section, we see that the 3 and 2 alternate. We're missing the 1 from the simple bracket, but if you continue across the stair, you'll find that it basically moves the bracket over each time rather than repeating in the same place.
This means anything with brackets will help your fingers get used to the patterns in double stairs. You don't need to focus on maps with lots of double stairs unless you want to get an idea of how well you're actually improving at that specific pattern.
