So what's a good way to get proficient? How can I train myself to use two fingers effectively?
— Start with the open strings.
— Then move to scales and arpeggios.
— Then move to patterns with string jumps (i.e. from E to D, E to G, A to G, G to E, etc.)
Using a metronome to measure your progress is important. For instance start at a very slow tempo (40-60bpm) and listen carefully for evenness of tone. This is one of the hardest things to achieve, each finger will sound a little different because they don't strike the string at the exact same spot and the amount of “meat” on the string may vary. Practice plucking at various spots on the string, since the tension is highest plucking near the bridge and lowest when plucking over the end of the neck.
When you can play for 5 minutes with no mistakes, then move up to 80-100Bpm, then 120-140bpm, etc. Keep a log about your progress and record yourself (simple recording with your phone is fine).
Look at your plucking fingers at first, after a while STOP looking at them. Work on strict alternation, even when crossing to a higher string. When descending, use strict alternation until it’s time to shift to a lower string then use the same finger for the next note (a rake) and resume alternation.
A good exercise for practicing alternation is to play patterns with an odd number of notes, for example, play the first five notes of a scale, ascending only (i.e. C D E F G C D E F G, etc,). If you play the root with your index finger the first time, the second time you will use the middle finger. Practice starting phrases on a different finger, don’t always lead with the index. Practice patterns that use open strings.
The goal is to have total independence where it doesn’t matter which finger you start with and you don’t think about it any more. When I play, I only look at my left hand (if I need to, for learning a figure or for position shifts), never my right…my right hand just runs on auto-pilot. I shift from one finger (good for pumping roots or slow tempos) to two fingers (playing faster) without even thinking about it.