The basic principles of a novel differential resonant sensing architecture are explained, illustrated and experimentally validated. We describe how a digital mixer architecture may be built in order to enforce a mutually injection-locked oscillation state in two M/NEMS resonators. It is shown that, in this state, the phase difference between the two resonators is highly sensitive to mismatch and can be used to sense changes of physical quantities, whereas it is largely unaffected by drift (e.g. induced by unwanted changes in environmental conditions). Some results obtained with a PCB-mounted discrete-component mixer and two NEMS resonators co-integrated with an analog front-end validate this approach.