If the outside walls are open cell spray foam and you have decent clearance in the space above the switch box, you should be able to push some fish sticks down from above and get another wire in the box pretty easily. Even easier might be to fish down to the coach light as presumably it's only a couple feet below the ceiling of the attic space. Send a feed an neutral down to the coach location, drop the neutral at the light box, send the feed to the switch on the white of the coach s/l. You'd also have to run another wire to floodlight, so it would have a neutral as well.
Cheap/easy option, though: dummy switch at the two gang, switch the flood from the bonus space, and just make sure that the fixture itself has motion sensing capabilities. Keep the thing on all the time...