It is shown that synchronization is similar to amplitude stabilization: both mechanisms involve creation of harmonics and frequency reduction. Synchronization of LC-oscillators can be achieved using similar synchronizing circuits in each oscillator connected in parallel with the tank. The output of the first synchronization circuit is routed to the second oscillator and vice versa. This scheme provides coupling using first, third and other odd harmonics. In addition, introducing coupling between common mode outputs of the synchronizing circuits provides second and other even harmonic coupling. This type of coupling may be also used between common mode outputs of amplitude stabilization circuits. The benefits of using simultaneous first and second harmonic coupling were verified designing a low-power, low phase noise 5 GHz LC oscillator with quadrature outputs. The oscillator phase noise is lower than -121.2 dBc/Hz at 1-MHz offset over the tuning range with a power consumption of 1.8 mW achieving a FOM lower than -192.2 dBc/Hz.