The invention provides, in some aspects, a container system for medical solutions such as peritoneal dialysis (PD) solutions. The invention particularly features a system which includes a first compartment that contains a first medical solution, e.g., a PD osmotic agent, and a second compartment that contains a second medical solution, e.g., a PD buffer agent. The compartments maintain their respective contents separately from one another for purposes of transport, storage and/or sterilization. However, the compartments are fluidly couplable, so that their respective contents can be combined with one another, e.g., following sterilization of the agents and prior to their introduction into the patient's abdomen. To that end, a container system can include a diffuser that is disposed in a fluid pathway between the first and second compartments, e.g., to facilitate homogeneous mixing of the first and second PD agents. That diffuser is disposed within and moves relative to a structure, such as a port that defines the fluid pathway between those compartment. Thus, for example, the diffuser can comprise a body that “floats” within that pathway-defining structure and that moves from one end to the other (and/or to from points there between), depending on a direction of solution flow through the structure.