The cartridge with two rounds was called a duplex cartridge (not to be confused with a duplex load, which is a cartridge using two different powders). To the best of my knowledge, it was experimented with but never issued. (The military also experimented with triplex cartridges, which contain three rounds.)
The SPIV (Special Purpose Individual Weapon) was supposed to replace the M16 but never did. Actually the M-16 was adopted as an interim weapon till the SPIV could be perfected. I have no knowledge of the javelin shaped projectiles that you describe having been used in Vietnam.
Flechettes definitely were used there in shotguns and other weapons. They were shaped like javelins except they had tail fins. They looked very much like very small models of a rocket. These were discussed in some detail in a very recent post.