There are two broad categories of citywide human mobility, routine, composed of daily or periodic travel, and rare, which occurs during events such as the Olympic Games or natural disasters. State-of-the-art studies have shown that routine mobility patterns can be modeled stochastically, while rare human mobility modeling, essential to a variety of urban computing scenarios, such as emergency management and traffic regulation, is a much more challenging and understudied problem. Instead of training a rare-event-specific human mobility model, which suffers from the particularity of the rare events, in this paper we provide a new insight into rare events and propose a novel algorithm, CityCoupling, which establishes an intercity spatial mapping that uses human mobility in one city as input and reproduces human mobility in another city. More intuitively, we attempt to answer the question "What if this rare event happened in another city?". To find the optimal intercity spatial mapping, we utilize an expectation-maximization algorithm to estimate a probabilistic geographical correspondence matrix by regarding intercity trajectory matching as latent variables. Thereafter, a Gibbs sampling-based multiple hidden Markov model generates simulated trajectories. We apply our approach to a large mobile phone GPS dataset in Japan and determine the spatial mapping between Tokyo and Osaka to transfer the human mobility at the Great Eastern Japan Earthquake in Tokyo, which was heavily affected, to simulate what might have occurred if Osaka had been struck by the earthquake. We conduct the evaluation by assuming that New Year's Countdown is a rare event that occurs simultaneously in both Tokyo and Osaka, and thus we quantitatively compare our simulation with the ground truth in Osaka.