I control Vito, Thorn of the Dusk Rose and two 1/1 creatures with lifelink. I attack with these two creatures, my opponent doesn't block them and they deal combat damage to him. My question is if Vito's trigger ability triggers once or twice? I know that damage is dealt simultaneously, so maybe I gain 2 life as "one piece"? Or maybe it's separated? It doesn't make any difference, because in both scenarios my opponent loses two life, but I am curios what will happen here.
The ability triggers twice. Here's the relevant rule from the comprehensive rules, which also includes an example where it does matter whether it's once or twice:
702.15e If multiple sources with lifelink deal damage at the same time, they cause separate life gain events (see rules 119.9–10). Example: A player controls Ajani’s Pridemate, which reads “Whenever you gain life, put a +1/+1 counter on Ajani’s Pridemate,” and two creatures with lifelink. The creatures with lifelink deal combat damage simultaneously. Ajani’s Pridemate’s ability triggers twice.
The Gatherer pages of at least some of the cards where it matters also include this in their Rulings section, e.g. under Vito, Thorn of the Dusk Rose it says
Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, Vito’s first ability will trigger twice and you may choose a different opponent for each trigger. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.
As indicated in that ruling, this issue can matter even for Vito if you have multiple opponents; another way it could matter is if your opponent wanted to cast Disallow to counter Vito's ability.