Alternating rear turn signals

One light above another on one side flashing alternately would not look like a directional. Confusing other drivers is not a good thing.

I reviewed a set of LED lamp bulb replacements that were guaranteed to have twice the lumen output of incandescent's they replace. I believe I would pick them any day of the week over alternating incandescent lamps for visibility. Same for adding a nice LED "third brake light" between the two lamps. The Mexican PFP (Federal Highway Patrol) has LED light bars when switched to "Hi" can be easily seen for 5 kilometers. A rhythmic strobe pulsing effect. Alternating flashing incandescent lamps would "look cute" but in my opinion do zero to aid safety or security.

You are chopping total lumen output in-half and lumens are what keeps the vehicle behind from eating your rear bumper.

My 1990 Bounder has them in front. The turn signal flases alternately with the front side light. Shows the vehicle next to me I need to come over. Sometimes it works and sometimes it doesn't. Usually the car right behind me floors it to get in front of me and sometimes they get run off the road(just kidding).

I'd be concerned about the legality of alternating flashers (even on the same side) on a non-official vehicle.... Yet I understand the desire to be seen.

This otter be fun to follow...

I think This place has one for a vet with only 2 tail lights on each side. Maybe you could use that one or one like it.

Problem. Your PRIMARY flasher does not stay on long enough for a secondary alternating flasher to work. The total timing event would have to be synchronized. Choose A, Choose B. Some automobiles used sequential tail lights years ago. The sequencers burned out like Roman Candles.

Google 12-Volt Alternating Flashers. You would need to think about one that alternates 3-times a second with a TRIGGER RESPONSE TIME of under 100 milliseconds. The alternating flasher MUST fire LED lamps. Standard incandescent lamp response time is far too slow.

Good luck, you bit off something that is going to need a lot of chewing.