Oh the wonders of the tail lights
I've thought of many, MANY things to do to mine, and having the outers run as the turns seems like a decent idea, mainly since they are both 3x57 base. I don't believe the turn signals (spliced in) would work in accordance to the outer brake lights tho since, if you hit the brake, they brighten up, and the alternating current from the turn signal might not disrupt the constant feed from the brake.
Plus you wouldn't want double the juice running to one bulb. That would just burn out immediately.
Although, if you wanted to splice the turn signal wiring and run it in-line with the inner tails, that would mostly likely work.
But back to the original plan, in order for this to work, I would:
1) Plug the turn-signal wiring into the outer brake-light housing. Idk why that wouldn't work properly, but that's the easiest thing I can think of.
2) Pull the outer brake-light wiring back into the car and somehow run an extension splice up through the wire loom (connected to swing arm for trunk) and plug that into the inner brake-light housing, so that would act as the dual-flashing brake-light.
As for what happens to the inner-brake light wiring, I'm not too sure about that one, since you can't just simply take the bulb out and let the wire hang, because the car will yell at you and say there is a light out. I guess you could splice in an LED strip somewhere and stick it somewhere, keeping in mind it will only light up when the headlights are on.