In this paper, we present the implementation and preliminary evaluation of a new type of silicon sensor for charged particle detection operated in Geiger-mode. The proposed device, formed by two vertically-aligned pixel arrays, exploits the coincidence between two simultaneous avalanche events to discriminate between particle-triggered detections and dark counts. A proof-of-concept two-layer sensor with per-pixel coincidence circuits was designed and fabricated in a 150nm CMOS process and vertically integrated through bump bonding. The sensor includes a 48×16pixel array with 50μm×75μm pixels. This work describes the sensor architecture and reports a selection of results from the characterization of the avalanche detectors in the two layers. Detectors with an active area of 43×45μm2 have a median dark count rate of 3kHz at 3.3V excess bias and a breakdown voltage non-uniformity lower than 20mV.