Is there a way to wield an offhand weapon or is it literally just a side arm?
Using two pistols of the same kind, you can use the abilities 1, 2, 3 and 4 of the pistol. First abilities use only one pistol. Last abilities may use both pistol.The system is not great.
You would expect to use both pistols anytime (at least for basic abilities).
If you can't, it would be better to cross abilities : having first ability of each pistol on the mouse and the second on the keyboard.
I have found running las pistol and auto pistol is a great combo also the plasma with the auto or bolt pistol is fun too