Melchizedek... was both a king and Priest and was the King of Salem (Jerusalem).
This was during the time of the war of Abram with the Nine Kings.......!
After the Law, It was against the LAW for a King to be a priest or a priest be a king... For obvious reasons these two positions were separated. That is until JESUS. Of course the LAW and the Prophets were until John (the Baptist) or the OT.
There is another Person(s)-group that are declared by Jesus to become Kings and Priest. This is the 'Body of the Jesus Christ' or His Church (Bride to Be). As we see in Rev. 5:9-10, the 24 elders were singing a NEW song Glorifying GOD and identifying themselves to be:
v10... "And hast made us unto our God kings and priests: and we shall reign on the earth."
So we have Melchizedek, Jesus and the Bride who are both Kings and Priests.
Hope this helps.. Hope you have a Blessed evening
Blade