It makes sense that since we've got two protagonists that share pretty much equal billing, their stories will have a lot of symmetry, even if the details are totally different. This works out pretty well as far as their love lives are concerned: both get a primary and a secondary love interest. We're meant to work out what their totally opposite ways of relating to these love interests say about them.
Becky is mostly a tease, but she makes that work for her and ends up with a couple of guys who worship the ground she walks on. First Rawdon, the hot jock who ends his macho career and drops all his other girls for the chance to marry her. And then Jos, the fat semi-loser, who is equal parts scared of and in love with her. Sadly for both of them, Becky is too much of an actress and too dedicated to getting ahead in life to really commit to an emotional attachment. She uses the men in her life until they have nothing left to offer.
Becky's m.o. is to play hard to get, but Amelia's is the opposite. She's all in or she's all out, and either way she can dedicate her whole emotional existence to maintaining the relationship. So she ends up with two men who can work with that. There's George, the conceited stud who takes her for granted and is embarrassed by her (since she sends long, mushy love letters to him at his club). Then there's Dobbin, the awkward but secretly awesome guy whom she totally sends to the friend zone. Amelia seems to get the better deal, since she ends up with Dobbin, but then again she spends most of the novel trying to love the portrait of a dead guy, so you make the call.