A system for separating fluids by centrifuging includes a rotor assembly for rotating in a centrifugation device. The rotor assembly includes a first rotor for housing at least one tube, a second rotor having at least one collection compartment, and at least one conduit. Each tube may contain a sample for separation by centrifuging, each compartment corresponds to a particular tube, and each conduit may provide fluid communication, upon demand, between a particular tube and a particular compartment, so as to enable transfer of a fraction of each sample from a particular tube to a particular compartment.