Unfortunately, the XP-67 prototypes were never fitted with their planned armament before being scrapped, and so far, I haven’t found any source that mentions this.
Since the XP-67 was designed as a bomber interceptor, one can assume that its six 37 mm M4 cannons were intended to fire simultaneously, delivering devastating firepower against enemy bombers.
There was a photo of the XP-67’s cockpit. It showed a control yoke with a missing firing button on the left and a microphone switch button on the right. This reinforces my hypothesis that guns only fire at the same time by pressing a button on the yoke.