You should make the catapult and the battering ram vehicles, not items, but make them move very slow (like SLOW).
Like.
One player should be at one mount point to move either vehicles. But on the battering ram, a player should be at one mount point to control the ramming (or two players mounted at separate mount points next or across from each other to enable the ramming).
But the catapult's firing techniques is a different deal.
SIDE NOTE: ramming?