Sorry to necro.
Trying to do this mission again here in 2018 and after 50 - 100 attempts i just can't get it.
Back in 2016/17 it just seemed to be a case of simply assigning target A to a couple of members and B to a couple of other members and then selecting open fire once Slingshot finishes his 'Light them up' line.
Both targets would drop simultaneously as if scripted or something and bob's yer uncle.
Now multiple shots are fired and my team never seem to be able to hit both guys at the same time.
I suspect that over the years some patch or addition might have broken this mission.
Anyone that has had past success able to have a go at this and report back?
If you could have 2-3 tries to prove repeat-ability that would be appreciated.
Back in 2016 everything i read above matches how it once worked for me.