use a canti/center pull brake cable hanger and yoke... there are positive locating yokes that use set screws to firmly locate the yoke in a set position... the Cyclocross i i maintain uses two of these yokes...

Tektro RBP-1246A RBP Bike Cycling Brake Yoke - Black

TEKTRO FRONT SILVER BICYCLE BRAKE CABLE HANGER (249)

there are other configurations available of the cable hangers... and cable STOPS that clamp onto frame tubes, too....

run a single cable to the hanger and yoke, cut another shift cable in half, thread through the yoke, run the ends to the derailleurs... adjust as needed! One lever, two derailleurs operated!

best to use two identical derailleurs...... and i'd think a friction shifter would be best to use... less tuning issues that way... index(click) shifters would be possible, but will demand far more set-up patience....... also, i'd recommend using all new stranded/lined shift cable housing, of all the same manufacture....

good luck with your project! report back with pics of the final machine, ok?