We demonstrate an efficient simultaneous self-injection locking of two semiconductor lasers to high-order modes of a standalone monolithic non-confocal Fabry-Perot cavity. The lasers are used to generate a low-noise microwave signal on a fast photodiode. The overall improvement of the laser spectral purity exceeds 80 dB. The observed single-sideband phase noise of X- to W-band signals is at the -110 dBc/Hz level and is limited by the fundamental thermorefractive noise of the cavity. The demonstrated cavity-laser configuration can be tightly packaged and is promising for the generation of high-frequency RF signals as well as for referencing optical frequency combs.