How do Pitiless Plunderer and Bontu's Last Reckoning interact with each other?
Yes.

Triggered abilities trigger once per event, and the trigger conditions defines the event. 

"Whenever a creature dies" will trigger every time a creature dies.

Since Pitiless Plunderer is phrased similarly to this, it triggers twice.

"Whenever one or more creatures die" will trigger every time one or more creatures die simultaneously. 

Since Pitiless Plunderer isn't phrased similarly to this, it doesn't trigger only once.