In Mg and some of its alloys, twinning and detwinning play an important role in their plastic deformation at room temperature, because their hexagonal-close packed (HCP) structures have a limited number of activated slip systems. Between two twins that are close to each other, it is known that there exists a critical spacing less than which their interactions become non-negligible. However, the influences of the twin thicknesses have not been investigated. Here, using a fully-field, crystal plasticity-based fast Fourier transform approach, we explore the interactions between two twins of parallel or non-parallel in Mg and its alloys while systematically varying the twin thickness. By quantifying these twin-twin interactions, our results provide insights into the underlying mechanisms of deformation twinning in HCP metals and alloys.