yes. tons of split pcb's on the market all support dual promicros or similar controllers. firmware is already written, just dump in your layout, pins, keymap, etc. and it's done.
i've done it for a couple one-of's others have built here on the forum. between, KLE, the firmware, VIA and a spare promicro i can troubleshoot the layout/firmware without actually building it. VIA does a good job representing the layout in the firmware visually and is usually pretty easy to catch any goofs on your 1st "make" of a new layouts firmware.