JOHN - In Joseph Smith History, it indicates that when John the Baptism confired the Aaronic Priesthood to Joseph & Oliver, he told them to baptise each other, which I understand, but, why did they ordain each other to the aaronic priesthood when they had just received it?

JOEL - Here are the relevant scriptures that record the event:

68 We still continued the work of translation, when, in the ensuing month (May, 1829), we on a certain day went into the woods to pray and inquire of the Lord respecting baptism for the remission of sins, that we found mentioned in the translation of the plates. While we were thus employed, praying and calling upon the Lord, a messenger from heaven descended in a cloud of light, and having laid his hands upon us, he ordained us, saying:
69 "Upon you my fellow servants, in the name of Messiah, I confer the Priesthood of Aaron, which holds the keys of the ministering of angels, and of the gospel of repentance, and of baptism by immersion for the remission of sins; and this shall never be taken again from the earth until the sons of Levi do offer again an offering unto the Lord in righteousness.
70 He said this Aaronic Priesthood had not the power of laying on hands for the gift of the Holy Ghost, but that this should be conferred on us hereafter; and he commanded us to go and be baptized, and gave us directions that I should baptize Oliver Cowdery, and that afterwards he should baptize me.
71 Accordingly we went and were baptized. I baptized him first, and afterwards he baptized me—after which I laid my hands upon his head and ordained him to the Aaronic Priesthood, and afterwards he laid his hands on me and ordained me to the same Priesthood—for so we were commanded."(J.S. History 1:68-71)

There really was no explanation given by John the Baptist as to why they were to ordain each other again after baptism. The scripture simply says that they "were commanded" to do it this way (verse 71).
It is contrary to the order of heaven for those who have passed beyond the veil to officiate and labor for the living on the earth. This is why John the Baptist did not baptize them himself first. But he was able to confer the priesthood so they could baptize each other.
Probably the reason why they confered to each other the priesthood again, was to establish the proper order in the Church of baptising first then ordaining to the priesthood. No other explanation has been given for this that I know of.