It is doable, but the taps are very difficult to use with this high of a helix -- they take an extraordinary amount of force to push through, since the tap doesn't want to pull itself through a half inch in a single rotation. In order to make it work, you need to cut a much larger minor diameter than the actual thread just to have a chance at it.
I cut my multi-start ACME nuts on high-end cnc equipment in multiple passes. I tried making some custom taps, and all of them seemed to bog down fairly quickly. A custom combination roughing / finishing tap always seemed to be quoted north of $1000, so I had to find another way to do it. My method involves custom tooling and 5 helically interpolated passes with a thread cutter.
Ahren
www.cncrouterparts.com