Both loads are very high and lateral load which similer to each other in this perspective.as the chances of ocarence both is very low so it will unnecessorily increase cost of structures.though both occare it is not necessory to come in full intencity for which we design.so if both come but lesser than the design intencity a structure may withstand.a structure have certain level of failure according to is code it can be damaged but can be made useful after maintanance.a structure damaged but not collapse and another is collapse.so with a only one type of lateral force consideration a bulding can resist the deseaster but may be 1 two state of failure but not colupse and thus we can save several human life.
The probability of occurrence of both earthquake and cyclone is very less.It is not possible to have both earthquake or a cyclone, or tornado at the same instant of time at the same place , unless it is an apocalyptic disaster which is possible only in case of movies.Hence we do not consider wind load and Earthquake load at the same time.
Yeah you have to consider only one factor in order to design because if we consider both,cost will increase and structure may not be subjected to that load in entire life