The mutual interaction of cross-equatorial twin cyclones, its effect, and the effect of the planetary vorticity gradient on their motion are investigated using barotropic nondivergent and shallow water models. In comparing the behavior of a single cyclone to that of twin cyclones, it is found that each cyclone tends to accelerate the other and to turn its track westward. The causes of these changes are illuminated by investigating the motion of a single northern cyclone in a mean flow that resembles the one induced by a southern cyclone. The meridionally varying environmental vorticity gradient induced by the southern cyclone plays a key role in the explanation of the northern cyclone motion. This gradient tends to cancel the planetary vorticity gradient around the northern cyclone, resulting in a lower effective ¿ that produces a weaker wave number 1 (WN1) asymmetry and consequently a slower motion. As the cyclone moves northward, the effective ¿ increases, and so does the WN1 asymmetric circulation, which in turn accelerates the cyclone. Even in the linear case, the mean varying vorticity gradient causes the WN1 circulation to be no longer east--west, but to turn counterclockwise. This effect is due to the development of a strong anticyclonic circulation that is at a maximum in the northeast region as a consequence of vorticity conservation. When the nonlinear effects are included, the ¿ gyres are turned further counterclockwise by the symmetric circulation, which is stronger than in the single cyclone case. Mutual interaction of tropical twin cyclones may increase the duration of the resulting Westerly Wind Bursts by reducing the poleward drift. The role of divergence effects on the twin cyclones' evolution is examined using a shallow water simulation with the same cyclone wind profile as in the nondivergent case. It is found that divergence tends to slow down the cyclone motion. We suggest that this effect is due to the potential vorticity conservation (compared to absolute vorticity conservation for nondivergent flows) that tends to reduce the produced WN1 asymmetry.