It is shown numerically that two fundamentally different regimes (I and II) of isochronous synchronization are possible, depending on the detuning between the lasers and the strength of the optical injection into the slave. We consider a master laser subjected to isotropic optical feedback and which is optically coupled, in a unidirectional way, to an open-loop slave laser. A master laser which exhibits a dominant x-LP mode and a weak y-LP mode was chosen. The x-LP mode was selected, rotate it to y direction, and optically inject it in the slave laser, which, under free-running conditions, emits x-polarized light.