:

Floating ground = shared, at least in my
experience.

On my early '80s GM stock stereo, there
were speaker outputs for four speakers
as follows:

Left-Front Positive, Left-Rear Positive,
and a single Left-Minus(both left speakers
Minus wires tied into that). Ditto on the
Right. It was called 'floating' ground
because there weren't individual negative
leads for the front and rear speakers
on each side.

Probably done to save space and money
by using less copper.