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user89071
You can use the existing 2 speed shifter by following the instructions from the author of this post: dmcg. The middle gear (cog) is going to be indexed into the small indent that you cut into the Brompton shifter.
I had better luck with an older Suntour friction thumb shifter. One that would be used to shift the front derailleur (left side shifter). The friction mode is easy to hit the middle gear even on the closely spaced 10 speed cogs. Any friction shifter for a front derailleur will work.