I think there is more than you think that can be done concurrently. It's true that early on there is only one cube to share, but once you get past hand-feeding and start automating delivery of the cube things start to open up. I've played with friends and not really had this issue.
One player could work on logistics system for the cube while another builds factories to turn intermediates into end products. Or different players could handle different cube processes and join them together. I do think multiplayer will work best with collaboration on voice chat so you can puzzle over designs together.