An assay method and device can perform at least one (e.g., at least two) assays on a single aliquot of a sample liquid. The device can mix a sample liquid with assay reagents including magnetically susceptible particles. The device is configured to create a sample liquid-air interface with the sample liquid. The magnetically susceptible particles can be located (via an applied magnetic field) at the liquid-air interface when a second liquid contacts the interface to form a liquid-liquid interface. The magnetic particles travel across the liquid:liquid interface to the second liquid. The magnetically susceptible particles are configured to transport an analyte across the interface into the second liquid. An assay for the analyte is performed in the second liquid. An assay for another analyte can also be performed in the sample liquid.