(500) Days of Summer does much the same thing. We know from the start that the couple in question isn't going to end up together. The film goes mostly chronologically with two starting points: the giddy beginning of the relationship and the post-break-up losses. The two storylines are intercut together to create the sort of perspective that only a bigger picture can give.
While Two For the Road, I feel, gives us a better look from the romantically-minded female perspective on the relationship, (500) Days of Summer gives us the romantically-minded male perspective. Joanna and Tom both fall in love quickly and fully invest themselves into their respective relationships. Maybe they should get together so neither of them will be let down by their less invested counterparts.
Tom (Joseph Gordon-Levitt) and Joanna (Audrey Hepburn) enjoying their road trip together as their car starts to catch fire.
Incredibly, the two films end on hopeful notes. Two for the Road ends with Joanna (Audrey Hepburn) and Mark (Albert Finney) accepting that they're a permanent fixture and complement each other well (exchanging the magical "Bitch" and "Bastard"of affection). (500) Days of Summer shows us Tom (Joseph Gordon-Levitt) finding peace with Summer (Zooey Deschanel) marrying another guy and starting over again (giving us a fun 4th wall-breaking look to the camera).
With humor and retrospective wisdom, these two films give a larger picture of what a relationship is like--not just an anticiption of one--and even tell us it's worth it. Now if only real life would allow me to dance to Hall and Oates in the middle of the park.