Dynamic cantilever sensors have many applications, for example in material's research, biology, as gas and magnetic field sensors. The sensing principle is based on the effect that a force gradient or mass change applied to the cantilever alter its oscillatory state which can be related to the parameter of interest. In order to detect very small interactions, the cantilever needs to have a low stiffness which is commonly achieved by a reduction of the beam's dimensions, especially its thickness. However, this is limited by the commonly employed laser-based detection of the cantilever's oscillatory state. In this paper, we describe a novel co-resonant cantilever sensor concept which is based on the coupling and eigenfrequency matching of a micro- and a nanocantilever. This approach allows to access a large fraction of the nanocantilever's high sensitivity while ensuring a reliable oscillation detection with standard laser-based methods at the microcantilever. Experiments in cantilever magnetometry and magnetic force microscopy demonstrate the immense potential of the sensor concept. Furthermore, applications are not limited to material's research, instead this concept creates a cantilever sensor platform with many potential applications, for example as gas, mass or pressure sensors.