when the lizardmen salamander shoots, it puts the flame template to it's mouth and then roll the artillery dice to see how far it overshoots.
Let us say a unit is engaged in combat with another unit. Behind those two units there is another unit. Can I target the unit behind those engaged in combat, even if it might land on them?
On the cannon it says that it is forbidden to shoot in a way that might end up bouncing or landing in a unit engaged in combat, the catapult might scatter into combat, the flamethrower however, doesn't say anything about it, nor does it say anything in the lizardmen army book.