The two lineages are literally just two SNPs apart. SARS-CoV-2 averages ~1/3 of an SNP per human-to-human transition, so ~1/9 of such transmissions generate "two lineages" at least that different. So intuitively, it seems easy to believe those lineages could have evolved in just a few weeks of early cryptic (unsampled) human spread.
Pekar et al. did some complicated modeling that purports to establish that the MRCA in humans is so recent that the two lineages must have arisen in animals, implying two introductions into humans. I believe that's highly suspect though, per my explanation and links at
Pekar et al. did some complicated modeling that purports to establish that the MRCA in humans is so recent that the two lineages must have arisen in animals, implying two introductions into humans. I believe that's highly suspect though, per my explanation and links at
https://news.ycombinator.com/item?id=32740568