The use of pairs in dreams is drawing attention to the idea of duality. We are trying to achieve a balance between the spiritual and material realms.
Psychological / emotional perspective: A dream clarifying the masculine side of ourselves may be followed by a dream clarifying the feminine.
The juggling that goes on in this way can take place over a period of time. Often in dream interpretation looking at the opposite meaning to the obvious can give us greater insight into our mental processes.
Material aspects: The unconscious mind appears to sort information by comparing and contrasting. Particularly when we are aware of conflict within ourselves, we may dream in pairs (e.G. Old / young, masculine / feminine, clever / stupid). It is almost as though there is some kind of internal pendulum, which eventually sorts out the opposites into a unified whole.
Gives gender - specific: It is quite natural for us to seek partnership, and in dreams both men and women will dream of duality as a way of sorting out emotional and spiritual complementary aspects. You might like to consult the entries for numbers and partner / partnerships for further clarification.