A hollow inner plunger for use within a syringe-in-syringe mixing system for mixing a two-part dental composition. The hollow inner plunger includes a body having a continuous cylindrical wall defining an internal chamber for containing a first component. The body includes a proximal end and a distal end. A sealing plug and rupturable membrane are disposed at the distal end of the body, and the sealing plug and rupturable membrane are integrally formed together as a single piece (e.g., formed of a single piece of elastomeric material). An associated syringe-in-syringe mixing system includes a first plunger, the hollow inner plunger, and a syringe barrel configured to contain a second component. When assembled, the first plunger is slidably disposed within the hollow inner plunger, and the hollow inner plunger is slidably disposed within the syringe barrel. The two components are initially separated by the rupturable membrane.