This work describes a new type of integrated lab-on-a-membrane foldable device suitable for on-site duplex electrochemical biosensing using drop-size sample volumes. The devices are fabricated entirely by screen-printing on a nylon membrane and feature two assay zones which are located symmetrically on either side of a three-electrode voltammetric cell with a bismuth citrate-loaded graphite working electrode. After the completion of two spatially separated drop-volume competitive immunoassays on the assay zones using biotinylated antibodies labeled with streptavidin-conjugated Pb- and Cd-based quantum dots (QDs), respectively, the QD labels are dissolved releasing Pb(II) and Cd(II) in the assay zones. Then, the two assay zones are folded over, and they are brought in contact with the voltammetric cell for simultaneous anodic stripping voltammetric (ASV) determination of Pb(II) and Cd(II) at the bismuth nanostructured layer formed on the working electrode by reduction of the bismuth citrate during the preconcentration step. The fabrication of the devices is discussed in detail, and their operational characteristics are exhaustively studied. In order to demonstrate their applicability to the analysis in complex matrices, duplex ASV-QDs-based determination of bovine casein and bovine immunoglobulin G is carried out in milk samples yielding limits of detection of 0.04 μg mL(-1) and 0.02 μg mL(-1), respectively. The potential of the devices to detect milk adulteration is further demonstrated. These new membrane devices enable duplex biosensing with distinct advantages over existing approaches in terms of cost, fabrication, and operational simplicity and rapidity, portability, sample size, disposability, sensitivity, and suitability for field analysis.