FWIW, I've just been reading a mandolin-building book (quick plug for "The Mandolin Manual" by John Troughton - I think it's worth reading if you're a novice) and they describe a mando-bass with 4 courses of *unison* strings. No plans, but confirmation that such a critter exists. An alternative arrangement, perhaps?
One thought with the headstock - could you move the tuners to the body and use a headless design? May be easier to get it balanced properly.
One question though - how feasible is this thing to play? Bass strings are thick enough anyway, so playing two at once seems a bit of a challenge! :-) Unless you have a wide fingerboard and mostly play with the pads of your fingers.