We already announced that we will support chainable expansion boards in the next generation of the Duet series (project name DuetN^2G). Design work has already started. With Smoothie 2 taking so long, who knows, we may even have it first. Perhaps you want to be a beta tester when we have hardware available?
For the current generation board, I am considering supporting 2 additional external drivers connected to the CONN_LCD connector. We may manage 3, but after that, we start running out of processor pins. You could multiplex additional external drivers if you didn't want to drive them all at the same time.
Somehow missed that.. Glad to hear it should be coming though!
I would say I'd be happy to be a beta tester but it's likely to be a couple months before I actually build the frame out so I don't really have a good way to be of use at the moment. I just have an off the shelf printer for the moment as to help with building the one I'm planning. If I do get it built before the new hardware is ready then I'd be happy to help! Do you have any guess on timeframe? I'm still in the early stages of planning what exactly I'm going to build so I'm not really in a hurry, just more curious.