Borderlands TPS: How to duelwield two different guns as Nisha (PC ONLY)

I found out a way to Duelwield any gun, and a pistol in my offhand. Works in the same fasion as gunserking (the passive effects from the offhand carries over to the main gun) so, if anyone finds out how to put mission weapons in your offhand, please let me know via a video response. Thank you for watching, and i'll (hopefully) see you later