Unfortunatly i can´t tell you how to fix this (i have the same problem atm), but i can tell you why. A normal person would think, that the first input-hardware (mouse+keyboard) controls the first Character, the first XBone/PS4 (do X360 and PS3 Controler work on PC?) Controler controls the second character, the next controller controls the third player etc.
But for some reason Epic thought “hey, let´s link the first input type to the first character”. That means, that the first character receives the input from the first Keyboard AND from the first controller. That means that you need 2 Controllers for 2 Characters, even if you control the first character with a keyboard.