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.
The percussionist Trilok Gurtu has said the same thing about listening to many recordings as he was growing up. He just assumed that all the percussion was played by one person, all at once, and so he figured out ways to do it, even when it was 2 or even 3 people with overdubs.
He tells it everywhere. (Also demonstrates the thumb vs fingers playing independently everywhere.)
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