If player A controls two (2) Pandemonium and player B controls Leyline of Combustion, how would this resolve. Player A has a creature enter the battlefield and has it deal damage to player B twice using each Pandemonium trigger. Does player B have Leyline of Combustion deal 0, 2 or 4 damage to player A?
The main concern is does the creature target the player.
The Leyline deals 2 damage twice, for a total of 4 damage.
The current oracle text of Pandemonium's ability is:
Whenever a creature enters the battlefield, that creature's controller may have it deal damage equal to its power to any target of their choice.
This ability has a target, so it can trigger Leyline of Combustion's ability.
The exact sequence of events looks like this:
The creature enters the battlefield under player A's control. Each Pandemonium's ability triggers.
Player A puts one of the Pandemonium triggered abilities on the stack and chooses player B as the target. Player A controls the ability and player B is the target, so player B's Leyline's ability triggers.
Player A puts the other Pandemonium triggered ability on the stack and makes the same choices. Player B's Leyline's ability triggers again.
Player B puts each of the Leyline triggered abilities on the stack.
Each of the Leyline triggered abilities resolves dealing 2 damage to player A, for a total of 4 damage.
Each of the Pandemonium triggered abilities resolves, making the creature deal damage equal to its power to player B.
As Murgatroid99's answer says, Leyline of combustion will trigger twice, doing 2 instances of 2 Damage.
The reason wording is the way it is on Leyline of Combustion is because of cards like Arc Lightning or Fireball spells which target more than one thing all at once - though you are targeting more than one thing that would trigger Leyline of Combustion in these cases, the targeting happens all at once, so the Leyline is only triggered one time. In the case of the Pandemonium here, the two Pandemoniums are separate, each going onto the stack and targeting separately, and each triggering the Leyline.
If Leyline of Combustion had said whenever you or a permanent you control becomes the target, it would count each target, not each instance of targeting, and Arc Lightning would trigger it up to 3 times.
