Leyline of Combustion w/ two Pandemonium triggers
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.