First thought is
assuming you mean that you have two weapons that you want (on a key/mouse press) to fire weapon a for one shot then weapon B for the next shot.
You could do it with a flag/counter/variable that is set to "left"
when the gun is fired then gun left routine is called and at the end of this routine the flag is set to "right"
so when the gun is fired again it will call the fire right routine
so in pseudo code it would be something like
on mouse pressed
--if "left" then -- spawn left bullet at left location : set flag to "right"
--else -- spawn right bullet at right location : set flag to "left"
not tested but sounds right - I will try to put up a capx soon.