We studied the dynamic behavior of laser-diode-pumped nonidentical thin-slice solid-state lasers, coupled face to face, with orthogonally polarized emissions. When such an incoherent mutual optical coupling was introduced, the coupled lasers exhibited slow fluctuations of transverse-mode patterns, and isolated lasers exhibited stable transversemode-patterns. When one laser exhibited nonorthogonal multi-transverse-mode operations without coupling, simultaneous random bursts of chaotic relaxation oscillations took place in both lasers over time with coupling. A plausible physical interpretation is proposed in terms of the simultaneous excitation of chaotic relaxation oscillations in both lasers through resonances that stem from interference-induced modulation of one laser at a swept beat frequency of the fluctuating nonorthogonal mode pair. Observed instabilities were well reproduced by numerical simulation of the model equation.