Acomodan comes from the verb acomodar, which means to fit, settle, or arrange. It's a versatile word that goes beyond simple placement, implying a sense of things falling into their proper or comfortable position.
In
"Segundas Partes" is Grupo Firme talking themselves out of a breakup and failing. The singer tries logic first: "Ayer convencí a mi orgullo" (yesterday I talked my own pride into it), sitting down with his ex to hash things out, and they land on the same verdict anyway: "Que un 'Sin ti' / Ya no soy yo" (that a "without you" means he isn't himself anymore). The whole song is really a rebuttal aimed at whoever coined its title phrase, "A ese que dijo que las segundas partes no funcionan" (to the one who said second chances don't work), because for him the cliché just doesn't hold: "Jamás lo había besado una persona como tú" (no one had ever kissed him the way she does), and now "Sin ti las piezas no se me acomodan" (without her the pieces won't fit back together).
He has a comeback ready for the other standard breakup line too. "Que olvidar es fácil, lo dicen por su falta de experiencia" (forgetting is only easy, he says, for people who've never actually lost someone), and the chorus keeps circling back to one question instead of moving on: "El corazón pregunta si regresas" (the heart keeps asking if you're coming back). It ends on "Y la bajada como que le cuesta" (and coming back down to earth seems to cost him), a plainer way of saying he's still stuck up there, hoping.