Tommy Emmanuel apparently learned by transrcibing, famously thinking that both the bass line and guitar lines he was hearing were a singular "guitar part". Just by having his expectations (incorrectly) raised, he rose to the occasion and played both parts.
I forget where I heard this story -- it's probably either rather famous, or buried in an interview somewhere.
loading story #47686128
Tommy drew a lot of inspiration from Chet Atkins who was really the pioneer of the bass+guitar "one hand band" style of playing. Tommy just improved on it a lot, adding more rhythmic elements, but to your point, yes, he was largely self-taught and driven to learn.
A good interview on his background: https://www.youtube.com/watch?v=py4T1qv9bnQ
He tells it everywhere. (Also demonstrates the thumb vs fingers playing independently everywhere.)