Self-seeding is a process unique to polymer crystals, which consist of regions of different chain packing order and different crystallinity. Here we report the synergistic self-seeding behaviour of pairs of core-crystalline block copolymer (BCP) micelle fragments and show how this strategy can be employed to control the morphology of these BCP comicelles. Each micelle fragment has a critical dissolution temperature (Tc), and unimers of each BCP have a characteristic epitaxial growth rate. The Tc value affects the dissolution sequence of the fragments upon heating, while the unimer growth rate affects the growth sequence upon cooling. By carefully choosing micelle fragments having different Tc values as well as growth rates, we could prepare patchy comicelles and block comicelles with uniform and controllable length. This synergistic self-seeding strategy is a simple yet effective route to control both length and morphology of core-crystalline comicelles.