If you control Panharmonicon, do two enter-the-battlefield triggers each trigger twice or just once each?

Panharmonicon states that if an artifact or creature entering the battlefield causes a triggered ability to trigger, that ability triggers an additional time. This effect applies independently to each triggered ability that meets the condition.

Under CR 603.2, triggered abilities trigger whenever their trigger condition is met. Panharmonicon's replacement-like doubling effect (CR 614) causes each qualifying ETB trigger to go on the stack twice — once for the normal trigger and once as the additional trigger Panharmonicon grants.

If you have two ETB triggered abilities (say, from two different permanents both entering, or one permanent with two separate ETB abilities), Panharmonicon applies to each of those triggers individually. Each one triggers twice, resulting in four total triggers on the stack.

Example: You control Panharmonicon and cast Mulldrifter. Mulldrifter has one ETB trigger ('draw two cards'). Because of Panharmonicon, that single trigger triggers twice, so it goes on the stack twice and you draw four cards total. If you also had a Soul Warden on the battlefield, its ETB trigger from Mulldrifter entering would also trigger twice, gaining you 2 life.

So with two distinct ETB triggers firing simultaneously, Panharmonicon doubles each one separately, giving you two instances of each — four triggers total. They are placed on the stack in active-player-controlled order per CR 405.3 and 603.3b.