You can increase your damage output by mapping a weapon to your shortcuts and firing with two or even three buttons at the same time. Jason has a hard limit on both number of shots on screen and number of shots before cooldown, which nerfs this trick considerably. You can achieve good results by mapping certain guns to the shortcuts which are different to the gun on the fire button (Wave and Diffusion, for example).

Sophia can still fire as fast as you can spam with this method. When using the crusher shots, a max of three blasts can be on the screen at any time. Anti SF Ammo is not counted for this limit, although a max of one ASF Bullet can be on the screen at any time. Rain Cluster cannot be combined with the other main weapons in any meaningful way. Something interesting does happen if you try to use Rain and Crusher at the same time though.