PlayStation: hold Triangle + press R1 for right-hand weapon, or hold Triangle + press L1 for left-hand weapon
Xbox: hold Y + press RB for right-hand weapon, or hold Y + press LB for left-hand weapon
PC: hold the two-hand modifier key and press the attack button for the hand you want to two-hand
Open the equipment menu and equip a weapon in the hand you want to two-hand
Make sure your off-hand is empty if you want to two-hand that weapon more easily
Use the two-hand input again to return to one-handed mode