Enable Tap to Shoot and you can shoot when aiming by tapping without having to bother with the shoot button. You can do the same with jumping to slide/dodge easier.
I know. That doesn't solve the problem though. You still can't aim and shoot at the same time, you have to lift your finger and do a separate action. Just like with the button, only with the convenience of doing anywhere on the screen. In fact, it's somewhat worse, because you have to tap for each shot, you can't even hold to keep shooting.