2 distinct sets of lights in night sky approching each other; alternating 3 flashing lights from left with 2 flashing lights from right

From passenger front seat, after midnight, I saw 3 flashing lights in the sky and to the right I saw 2 flashing lights approaching each other. I believed they were too close for helicopters - When 3 lights at left were on, the 2 from the right were off; when 3 were off, the 2 approaching from right were on....I immediately thought of something docking with something else (saw no craft) -- When the lights appeared to touch, they suddenly flashed as one, in sychronicity. Suddenly, object headed right toward and over mountains which would be to east, and it seemed to be out of sight in six seconds from where it was, unlike aircraft that my eye could follow for half a minute or so, (Only unidentified incident I ever witnessed) This was back in either 1975-77? I still wonder if it was military, but the speed....