1964.5 Coupe tail light question

Slowly starting the rewire project on a 64.5 six cylinder coupe. First question is about tail lights. Previouse owner bypassed the turn signal lever and ran a wire from rear tail lights to a toggle switch. It's a bit of a rat's nest under the dash as far as wiring from flashers. But my first question is about the tail lights. I look at mine and there are two bulbs in each housing. One bulb is for tail lights and one bulb is for brake/turns. Every wire diagram I've looked at shows one bulb. Do you suppose the P.O. ADDED a bulb to the tail light buckets as part of his work around (seems like alot of work if he did). Or was there ever a two bulb tail light bucket. Trying to decide if I should order new buckets and if they would be compatible with my wire harness. I do have a new tail light to turn signal switch harness. Mr. Mustang 64-65 tail lamp harness for coupe and convertible W/O boot sockets p.n. 1440564TLCWO.

Yeah, looking closer it is a P.O. work around. I quess he did not have brake lights (probably a harness problem) so the fix was to install a second light socket in the bucket. I see the sockets are new and "shiney". It has wire running between each light and then up to the switch on the brake reservoir. I quess that is where he is pulling power from ? Anyway I have a main engine wire harness, a firewall to headlight harness, a generator wire harness and a tail light to fusebox/turn signal harness, all new in the packages. In the fall I will begine a rewire project, everything except the under dash wire harness which I have inspected and it all looks good yet.