I have discovered a little useful method that I haven't seen mentioned elsewhere:
Just use two FCS keys and both hands at the same time to insert/remove FCS fins:

It works for sure better as an hammer, but what is the benefit of using 1 key on 1 screw at a time?

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Jeroensurf said..
what is the benefit of using 1 key on 1 screw at a time?

I guess you mean "the benefit compared to using 1 key" ?
- It is faster, especially with the "settling" phase that will require 4 key insertions total if only using one key.
- I find it satisfying to feel both forces at once. It helps me to gauge the amount of torque.
- It minimizes the inserting of keys, which must be done properly if you do not want to round it (their metal is soft on purpose)
- It is easy to do with no drawbacks.