Soul Warden's ability reads:
Whenever another creature enters the battlefield, you gain 1 life.
When Lingering Souls resolves, there are two separate instances of "another creature" entering the battlefield, and so the ability triggers twice, and you gain 1 life twice in two separate events. Thus, Karlov's ability will trigger twice, and he will gain four +1/+1 counters. Likewise, using Conqueror's Pledge instead will net you 6 life and twelve counters.
Compare rule 603.2c and its example:
603.2c An ability triggers only once each time its trigger event occurs. However, it can trigger repeatedly if one event contains multiple occurrences. See also rule 509.4.
Example: A permanent has an ability whose trigger condition reads, “Whenever a land is put into a graveyard from the battlefield, . . . .” If someone casts a spell that destroys all lands, the ability will trigger once for each land put into the graveyard during the spell’s resolution.