> This sort of notation is very common. Rather than notating only what
> is physically possible for the player, the notator tries to show the
> musical sense of the passage.
It's not actually even physically impossible: most instruments allow you
more than a binary tone-on/tone-off, and even with a binary
tone-on/tone-off, you have the choice to play notes with different
duration. In the case of the guitar, you would pick the note as loud
(and using the same picking fingers) as with the faster voice, but let
the note sustain in line with the slower voice while picking the next
note of the faster voice just as loud as the previous one.
"voicing" is an important and advanced skill for most instruments:
guitar, violin, pianoforte (much more so than with plucked keyboards
like harpsichord and spinett), accordion: the important thing is to lend
each _voice_ a consistent character (loudness, articulation) independent
from the distribution across fingers, hands, strings, and even given
colocation with another voice on the same notes. Of course, string
instruments have a bit more voicing possibilities by the decision which
string to take.
The violin works of Bach offer interesting voicings. A rather prominent
(and mostly monophonic but still multivoice) example is the first motion
from the violin solo partita 3: the voicing made very explicit in the
autograph can be straightforwardly mapped to different strings.