Described here are devices, systems, and methods for forming a fistula between two blood vessels. Generally, the systems may comprise a first catheter and a second catheter, which may comprise one or more fistula-forming elements. The first and second catheters may comprise one or more magnetic elements, which may be used to assist in bringing the first and catheters in closer proximity to facilitate fistula formation. In some variations, the magnetic elements may have magnetization patterns such that the flux generated by the magnetic elements is locally concentrated. In some instances, the system may comprise a magnetic control device, which may comprise a magnet, and may be used to increase or create an attractive force between the first and second catheters.