in combat after you declare your challenges because well chaos has to the remainder of the unit get's to assault the unit in the challenge.

senerio were chaos lord and world eater unit vs necron lord on a barge as well as chaos lord and berserkers vs a lone assasin. as far as i have read in 7th edition they challenge is fought at their inatitive step and the unit piles in at their inatitive step and takes their swings as well against both units they do not sit there with their thumbs in the air waiting for a ride. they being that they are a single unit and in combat they can and are targetable.