Sensing magnetic interactions at the atomic scale and utilizing those interactions for magnetometry have been highly challenging and important topics in the magnetic resonance community. Recently, electron spin resonance and scanning tunneling microscopy (ESR-STM) have been successfully combined, enabling spin resonance of individual atoms on ultrathin insulating MgO surfaces. When two magnetic atoms are positioned within the separation range of 1 nm–4 nm, two spectral features appear in the ESR measurement. The difference in those two frequencies follows a r−3 distance-dependence, indicating that the individual atoms are coupled through the magnetic dipolar interaction. Here, we discuss the spin relaxation times that lead to the observed ESR spectra. In addition, we suggest a quantum Hamiltonian model to obtain further insights toward, for example, studies of frustrated spin systems.