Why it's helpful to have two?
As described in the schema above there is one keyboard and a mouse shared by 4 computers. For the Pi zero, an USB / Ethernet adapter is added.
The USB shared hub is supposed to be powered by its USB connections. It works for the Mac mini but fails for the Pi zero.
Indeed, the shared hub draws 260 mA and the Pi zero draws 240 mA: both are too much for the Pi zero alone.
Definitely, it is better to have two, even for power supplies and USB multi-meters.