OK, sorry I was a bit vague with my descriptions. Anyway, I've experimented a bit and it seems that if I create two emitters in the same position but at different times, the particles from the second emitter appear in front of the other emitter's particles.
Screenshot:
Shows the effect of creating the emitters in a different order. In the second screenshot the fire is almost completely obscured.
Screenshot:
Shows the effect of creating the emitters in a different order. In the second screenshot the fire is almost completely obscured.