DJones did something similar to this on his TB. He bought the aftermarket tails that have clear lenses and all chrome reflectors in the back (super rare and almost impossible to find) and has 3 sets of switchbacks to have a few different functions use 2 bulbs on the same side.
The mod in the video wouldn't be very hard to accomplish at all. The reverse light sockets are 3156 (only 1 connection, using the high contact on a 3157 bulb) You would need to get a
pair of 3157 sockets (2 connections, using the high and low contacts) and of course the switchbacks you wanted.
Cut the turn signal wire from it's socket, do the same with both connections for the reverse light, take those ends and splice them to the pigtails. Reverse to the low side, turn signal to the high side, ground to ground. Pop in your switchback, test to make sure you got it how you want it, seal your connections, and enjoy!
Now if you have stock tails, then like Harpo said, you'd have to do some modding of the circuit boards instead, but the methodology is the same.