How do multiple copies of Merrow Commerce interact?
Well yes you can. These are triggered abilities


  603.1. Triggered abilities have a trigger condition and an effect. They are written as "[Trigger condition], [effect]," and begin with the word "when," "whenever," or "at." They can also be expressed as "[When/Whenever/At] [trigger event], [effect].


Multiple triggers are put on the stack, and after one resolves, you get priority as the active player.


  116.3b The active player receives priority after a spell or ability (other than a mana ability) resolves.