I use the UltraSlim docking with an HP EliteBook 840 G1 and an HP EliteBook 840 G3.
I can extend the image on the screens, which are connected to the 2 DisplayPort outputs, with both models.

I would like to ask if I can plug into the two display ports on the back and display 2 independent monitors with a 2013 HP Elitebok 840 G1 because there are models where you can only mirror DP but not get 2 independent outputs.

I don't understand the question....