My best guess for the answer to this question is that the listener in My Last Duchess interacts with the speaker while the listener in Life in a Love does not. In My Last Duchess, a rich duke is having a conversation with a man about a portrait of his late wife while negotiating a deal to marry a new woman. Life in a Love is about a man's reaction to a woman possibly leaving him. In the first poem, there seems to be a more active conversation going on. I hope this helps. None of the other answers seem as relevant, so I think this is the right one.
