basic assembler coop just reduces assembly for non-basic components
I turned coop mode on my basic assembler when I built my normal assembler. for the components the basic assembler could make this was great, for the rest it just cut the quantity in the master (normal) assembler in half.
the end result being that if I queued up 10 thruster components it would give me about 5, but if I queued up 10 steel plates I got them all (between the 2 assemblers).
I found this at the very end of the playtest and wasn't able to do multiple tests to check if it was a fluke or exactly how it affected the master queue, but it looked like half.
In case this makes any difference- it was done in single player in Never Surrender, all built on the hydrogen engine starter spaceship.