This paper describes a microfluidic chip in which two perpendicular laminar-flow streams can be operated to sequentially address the surface of a flow-chamber with semi-parallel sample streams. The sample streams can be controlled in position and width by the method of electrokinetic focusing. For this purpose, each of the two streams is sandwiched by two parallel sheath flow streams containing just a buffer solution. The streams are being electroosmotically pumped, allowing a simple chip design and a setup with no moving parts. Positioning of the streams was adjusted in real-time by controlling the applied voltages according to an analytical model. The perpendicular focusing gives rise to overlapping regions, which, by combinatorial (bio) chemistry, might be used for fabrication of spot arrays of immobilized proteins and other biomolecules. Since the patterning procedure is done in a closed, liquid filled flow-structure, array spots will never be exposed to air and are prevented from drying. With this device configuration, it was possible to visualize an array of 49 spots on a surface area of 1 mm2. This article describes the principle, fabrication, experimental results, analytical modeling and numerical simulations of the microfluidic chip.