The present disclosure provides a sub-assembly of a pump device comprising a housing comprising a chamber extending along a longitudinal axis. The chamber comprises an inlet and an outlet. Both the inlet and the outlet are open in a direction transverse to the longitudinal axis. The inlet is configured to be fluidly connected to a medicament container. The housing comprises a wall extending in the direction transverse to the longitudinal axis. The sub-assembly of the pump device further comprises a first piston and a second piston. Both the first piston and the second piston are at least partially arranged within the chamber and fluid-tightly engaged with the chamber between the inlet of the chamber and the outlet of the chamber. The first piston is lined up with the second piston along the longitudinal axis. The sub-assembly of the pump device further comprises a control mechanism configured to cause the first piston and the second piston to be selectively moved together or moved relative to one another.