NetKar does support 2 game controlers for sure, I have G27 and a DIY (mJoy) controller for my custom pedals and for a button box.
Go to Settings->Controller->Advanced and to the upper right there is a menu to select your joystick.
Select your logitech attack 3 joystick. Then, press Handbrake(menu at left) and then push the key you want for handbrake to your selected joystick.The key has a number that lights up at your right.Press that number and you have assign the action you want(handbrake) to your selected button.
I hope that I had understand what are you asking for. If not ,,,, just ignore,
Excuse my poor English....