A while back, near rivit city i was attacked by the group of mercs that show up there. I killed one of them and the other two were badly injured. With the a3-21 plasma rifle I shot from the hip at the head of one of them when he was bending down. He died, and so did the other one at the same time. It looked like a collatorel and there was no one else around, so I was wondering it is possible or is just a glitch...

With grenades, yes. Also with Gauss Rifle and other AoE weapons.. Also the Tri-Beam Laser, I believe. But I haven't heard of it happening otherwise.

Maybe he died of his wounds

You can not kill multiple people at once with a single shot weapon like the plasma rife with no area effect. With area effect or weapons with multiple bullets or beams like shotguns automatic weapons or special laser weapons like the three beam rifles and some unique.

Might be friendly fire, happens from time to time, know that people on your party hit each other from time to time then you have large groups.