Unless you plan to go full stiff, you're better off removing both from the car at the same time. The issue is that the rebound from a 1/2 turn (or whatever) on one strut is unlikely to match the same adjustment on the other -- it's inexact. If you have them both off the car at the same time, you can compress them at the same time, release, observe the rebound, and adjust accordingly. Maybe you wouldn't notice the difference in rebounds if you don't match them this way, but I was surprised how mis-matched mine were when I turned them exactly the same amount (before I installed them)