To answer your question.....yes they share the same filament on a 1157 bulb, the other filament is used a running/parking lights.
It is possible to have Brake/Turn and flashers with out using a column mounted turn signaling device........ by using relays and a couple of toggle switches it is very do-able.
I used a single electronic flasher, nothing special just one that was listed for a 67 Camaro.
I had to use a ON-OFF-ON toggle (DPDT), I couldn't get it to work without it because I needed to turn off the brake circuit when the turn signal was on.
I used a ON-OFF toggle (SPST) for the hazards
I used bosh style 5 pole relays (six total),there better relays for this app, but I used bosh style due to availability. I used some smaller ones that I salvaged from a dodge Dakota. All 6 relays taped together measure 1 ¾” by 1 ¾”
I assumed the brake switch would provide a +12v
How it works……..
Step on the brake (no turn sig) pwr is fed from the brake light switch to the 87a (NC) terminal on Relays 1a and 1b, out thru terminal 30 (Comm) into to the 87a (NC) terminal on Relays 2a and 2b, out thru terminal 30 (Comm) to the rear turn/stop lights……. None of the relays are energized.
Activate left or right turn signal (no brake light) intermittent pwr is fed from the single electronic flasher, thru the turn sig switch to the 87a (NC) terminal on Relay 3a or 3b, (depending Left or Right) and out thru terminal 30 (Comm), front turn light is also connected to terminal 30…… at the same time terminal 30 (Comm) provides power to the 87 (NO) and 86 (Coil) terminal on Relay 2a. Relay 2a activates and sends pwr thru terminal 30 (Comm) to the rear turn/stop light…… both front and rear turn bulbs light and flash….. due to the flasher. At the same time constant pwr is fed from a +12 volt source , thru the turn sig switch to the 86 (Coil) terminal on Relay 1a or 1b, the relay activates and disconnects terminal 30 (Comm).
Activate left or right turn signal and step on the brake….. basically the same as the turn, but when the relay 1a or 1b, activates and disconnects terminal 30 (Comm), which is the brake light feed, the brake light cannot illuminate….. hence the turn signal continues to flash, while the other side brake feed relay is not energized and the brake feed is still connected and the brake light remains on.
Activate the Hazards……. constant pwr is fed from a +12 volt source , thru the Haz switch to the 87 (NO) and 86 (Coil) terminals on Relay 3a and 3b. Relays 3a and 3b activate and flash all four lights…… the brake circuit is still in connected ….. so the brake will over-ride the turns and stay on.
looks and sounds complicated, but really wasn’t that hard to wire up.
I've attached a dia that is being used on at least two vehicles that I know of.