I've been thinking about this all day
The center wing would be easy enough to build, just tie the two spars together like you do with the HS spar.
The center leading edge would become one big tank, probably with more capacity than the standard leading edge tank.
Aileron push tubes would be easy to connect through the center wing.
Elevator control could be connected through the common elevator. (Note the loss of aerodynamic counter balances.) The trick would be to keep this thing in clean air.
Rudder control could be accomplished by putting a bell crank on each rudder and putting a push tube through the common HS.
Engine controls could be linked via push cables.
Engine and flight instruments are easier with the advancement of electronic EMS and EFIS systems.
Landing gear is the biggest issue. Retractable gear could be designed into the center wing. The tail wheels could be left hanging out in the wind like a standard -8 or they could be put in the tail like the guy from Germany did with his RV-4RG.
Fixed gear off the engine mounts wouldn't look very good, IMHO, and you would really have to put some baseball bat sized gear legs on it to hold up both engines.
Engines choice, if you are going to build this monster, you might as well put in tricked out IO-360's with three bladed props. Looks are everything after all.
Cost? I'm thinking it could be built for around what some are putting into their RV-10's. The low side should be around $130K and the high side up to $200K. (Oh, and don't forget to add in the cost of your divorce.)
Empty weight: 2300 lbs to 2500 lbs +/-
Cruise speed: 250 MPH +/-
ROC: 3,000 - 3,500 FPM +/-
Insurance costs: Uninsurable
(I really don't know about any of those numbers, just guessing.)
One question, where would you put in the second set of controls?
A. One set of controls
B. Both front seats
C. One side - front and back
D. One side - front and back, plus one front control on the other side
E. Both front and back on both sides