I can't help but think of this in viloin terms.

A description for those that don't know. A violin can normally
just play one note at a time. However, if you get *very* good
with your fingers you can play two notes at a time. There is a
story about Nicolo Paganini (an amazing 1782-1840 violinist) that
he once wrote a love ballad for a violin duet, however could not
find a partner to perform the other part. So he just played both
parts on the same violin at once! (sounds almost impossible to me!)
It is possible for virtuoso players to play three, and rarely
(songs must have special characteristics) four notes at once. More
than four is not possible, since at that point your out of strings!

Since the progression of 2 for a professional, 3-4 for virtuoso
matches exactly ... I think of the ability to play multiple songs
in these terms.

Two-at-once:
double-stop

Three-at-once:
triple-stop

etc...

Since my daughter is the violinist and I play woodwinds (which don't have
this technique), I couldn't help but think when my Bard first managed to
keep two songs up that:

Ha! This woodwind player can finally double-stop!

Starling
Working on her triple-stop (but finger not likely to comply)
Erollisi Marr