A major task for parents during the transition to second-time parenthood is to help their firstborn adjust to their new roles as siblings. Increased father involvement has been theorized to be protective for firstborn adjustment. Fathers, however, are under increasing pressure to balance both work and family responsibilities. Here we evaluate fathers' relative involvement in two-child families as a function of family structure, gender role beliefs, and work-family conflict in 222 dual- and single-earner families from the Midwestern region of the United States after the birth of a second child. Couples reported on father involvement with firstborns and infants when the infants were 1, 4, 8, and 12 months old. On average, fathers increased their involvement with infants but decreased their involvement with firstborns. Dual-earner fathers were more involved with their children than single-earner fathers. Although mean levels of father involvement were different between dual- and single-earners, multi-group parallel process trajectory latent growth curve models revealed more similarities than differences between dual- and single-earners in processes guiding father involvement. Both dual- and single-earner fathers engaged in juggling childcare between children and both dual- and single-earner fathers' involvement with infants was constrained by work-family conflict. Gender role beliefs predicted child care involvement for dual-earner, but not single-earner fathers: more egalitarian gender roles predicted greater involvement with the firstborn immediately after the birth of the second child. Results underscore the need for greater workplace support for fathers' caregiving roles after the birth of an infant.