Haskel now offers their EXT-420 plumbed so that you can run it in either single or double stage, depending on where volume or pressure are required. They did so using check valves and a single two way valve.
For ya'll who have the 59025-3 or standard EXT-420, here is how WolfWurx made them selectable using two three way valves. Two valves to switch instead of one, but ostensibly avoids the pressure losses that accompany spring checks.
Converted Haskel EXT-420