To clarify, it doesn't really make sense putting the Liberals and Nationals, since they don't run candidates against each other (although they should, since there's no vote-splitting).
Actually in some electorates they do.
Where?
In some National Party strongholds (New South Wales and Queensland). I think they even do that in one or two electorates down here in Victoria. Though those electorates are close to the border (Vic & NSW) one.
Do they do it for nationwide elections? I know they do in some states.