On a Twenty with steel rims you are going to need both brakes. I have the alloy rims on mine and I need both brakes, but I am pretty good sized at 6'-2" and 215#...
The original brakes on the Twenty are a bit on the flexible side and make for some weak braking power, but with Kool Stops it is still way better than what it came from the factory with 35+ years ago.
Aaron