I guess you will have to define more precisely what exactly "only have one thing shooting at one time?"
The action of shooting, is when the projectile is spawned.
It is likely that already currently your two different objects are not "shooting" at the very same time according to this definition.
So are you rather looking to only have a projectile at the time on screen (which would mean that only a single entity was allowed to create a projectile, and as long as the projectile is on screen, no entity can shoot again) ?
Clarify this point, consider possibly providing your existing project to allow more accurate investigation and you should get an accurate answer to your problem.