Select a follower on your field and an enemy follower on the field. Deal the first follower damage equal to the second's attack. Deal the second follower damage equal to the first's attack.
QIf I select a 0-attack enemy follower and a Servant of Disdain on my field with this spell, will Servant of Disdain's "whenever this follower takes ability damage, draw a card" trigger?
ANo. When an ability does 0 or less damage, no damage is dealt at all (it doesn't count as dealing damage).
QCan I play this spell only when I can select both a follower on my field and an enemy follower on my opponent's field?
AYes, that's correct.