According to Apple, yes you're doing the right thing. I disagree.
For usability, using two action sheets does nothing but wastes time while providing the illusion of cleanliness. It manages this by keeping the map background and the animation of the action sheet disappearing and reappearing upon selection. Considering your selection type, I'd suggest bringing up a new view specifically to guide users into following the path of your choice so they answer the question they're trying to answer.
In other words, bring up an overlay view with the first set of options, the dissolve the unselected options and display the second set of options. Then either highlight the second selection and provide a confirm function or just continue after the selection (I recommend the former, in case of error). Also make sure that users can back up; I recommend an edit button by the top selection, which will bring back the original options and dissolve the 2nd set.
That'll be $20.
Action sheets can work in tandem with each other but they are...heavy. Users have the data of the page behind and now additional options. It's cognitively aggressive. Keep it simple: show your questions one at a time with available options so users can follow a simple path, all while seeing where they came from. By using two action sheets the user can't see what the last choice was (leading to potential user uncertainty and error) and still leaves the map in the background, which is totally unnecessary.