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.