So it's very interesting, the description of how Lord Caitanya and Lord Nityananda are like the sun and moon appearing simultaneously.The nature of the sun is to light, the main nature of the sun is that it brings the light of day, whereas in relation to Lord Nityananda, the quality of the moon that is particularly described is the quality of being cooling.
The light of the moon is very cooling and it is said (Sanskrit) that the cooling effect of Lord Nityananda's lotus feet is like millions of moons that can cool down the burning agitation of the material energy upon the heart of the living entity. So in that way, Lord Nityananda in particular has this cooling effect!
We also see in the 7th canto of Bhagavatam how Lord Krsna is the one who gives transcendental knowledge, and then that it is Balarama who gives the strength by which one can follow transcendental knowledge. So in order to get spiritual strength, one first has to become calmed down - the agitation in one's heart has to cool down! Therefore Lord Nityananda helps us by cooling down the agitation in the heart.
(Kadamba Kanana Swami, CC Seminar 2003)
