PS4
If you fire off Holy Bolt quickly in one direction then Left Analog Stick (Movement) aim at a Mercenary and hit Normal Attack, it will fire a second Holy Bolt.
For example, I have Holy Bolt assigned to R1 and Normal Attack assigned to Cross.
R1, LS (Aim at Mercenary), X = 2 Holy Bolts.
You need to be within 10 - 12ft or closer to the mercenary.
Edit: I keep screwing around with this and changing my post to match what I find. I’m pretty sure this is exact now.