Using a scanning tunneling microscope (STM) combined with electron spin resonance (ESR), we performed electron-electron double resonance on two S = 1/2 Ti atoms on MgO surface. One (Ti(1)) was positioned under the tip, and the other (Ti(2)) was away from the tip and coupled with Ti(1) by a weak interaction . Quantitative understanding on the spectral features and underlying spin dynamics requires a combined study with simulations based on a model such as open quantum system. First, we observed splitting of each double resonance peak, which was linearly increasing with the driving RF voltage. We attribute these features to the AC Stark effect , directly providing the Rabi rates of both spins. With these Rabi rates as inputs, simulations of double resonance spectra within the frame of Lindblad formalism  let us estimate the energy relaxation times of both spins (T1(1) = 8 ± 2 ns, T1(2) = 150 ± 10 ns). In contrast to the fast relaxation of Ti(1) in the tunnel junction, the much longer relaxation of Ti(2) sheds light on possible use of such 'remote' spins as qubits on surfaces. In addition, we briefly discuss spin decoherence processes in this system.  Phark et al. arXiv:2108.09880 (2021);  Autler and Townes, Phys. Rev. (1955);  D. Manzano, AIP Advances (2020). *This work was supported by the Institute for Basic Science (IBS-R027-D1).