There is a message intended in these two. "And over it the cherubims of glory shadowing the mercy-seat; of which we cannot now speak particularly. " Paul had an understanding but was not ready to share it with those that were to receive the letter to the Hebrews. Maybe because they were not ready for it. However, if you have ears to hear it then consider the answer is in the phrase "cherubims of glory." The glory of God (often called the Shekinah, by the Jewish writers, though not in the bible) manifest in a cloud that appeared over the mercy seat and therefore between these images. They are shadows or types of the heavenly and we get more insight by reading Isaiah 6 where he sees the throne of God and the heavenly creatures singing Holy, Holy, Holy. They represent the Holiness of God that no man can approach unto until after the Cross and resurrection. Since Jesus has gone before us with his own blood he has made a way for us to enter into the very presence of God which we have not yet done physically but we are promised to be able to do so in the future. So when you see the similarly described creatures in Rev 4 singing Holy Holy Holy it is to remind you of Is 6 but as you read further in Rev 5 you see these creatures singing the following lyrics "You have redeemed us by they blood out of every tribe, nation, tongue, etc and we shall reign as kings and priests on the earth." They tell us that they are redeemed people and not cherubim even though they appear to John in similar appearance as the cherubim in Is 6 the message being that we will inherit that proximity to the very glorious throne of God that was once the place of these glory cherubims. Hallelujah! I can't wait.