The fact that Hal has been played by two different actors throughout the series is explained in this episode by stating that Maggie had married two men named Hal and has now gotten back together with the first Hal:
Paul: Hey. Is that-is that-
Maggie: My first husband Hal.
Paul: Your first husband. Hal.
Maggie: (emphasizing the lack of a pause in her phrasing) My first husband Hal. Both my husbands have been called Hal. My second husband Hal and I have separated, and my first husband Hal and I have rekindled. All right?