I started playing guitar when I was 11. I played for 2 years, but then I lost interest. I resumed playing 7 years later, when I was 20.
About 2 years after that I tried playing bass. Plucking the strings and fingering the frets wasn’t that difficult, but it wasn’t as easy as playing the 6-string. I tried to play it like a 6-string lead guitarist, so my playing didn’t have the drive that a good bassist does.
Most guitars have 6 strings. Some have 12, but the difference between playing a 6 and a 12 is less than the difference between playing a 6 and a bass. That’s because the 12 strings are mounted on the same neck as a 6-string, and the extra 6 strings are strung right next to the primary 6, so they don’t require a wider neck.
The bass guitar usually only has 4 strings. Those strings are thicker, and therefore require greater finger strength for plucking and for applying sufficient pressure on the frets. Also, I think a bass guitar neck is thinner, so there’s probably some adjustment to be made there too.
I think a bass guitarist could eventually play a 6-string, but the transition might be slightly more difficult. That’s because from what I’ve seen, most bass guitar work doesn’t require the ability to play chords, whereas that’s an essential skill for most 6-string guitar work.
In summation, it would be a lot easier for either guitarist to play the other guitar type than it would be for someone who’s never played, but a successful transition does require some work.