Thus in Psalm 103, we pray, "Bless the Lord, O you his angels, you mighty ones who do his word, hearkening to the voice of his word! Bless the Lord, all his hosts, his ministers that do his will!" (Ps. 103:20-21). And in Psalm 148 we pray, "Praise the Lord! Praise the Lord from the heavens, praise him in the heights! Praise him, all his angels, praise him, all his host!" (Ps. 148:1-2)
Not only do those in heaven pray with us, they also pray for us. In Revelation, John sees that "the twenty-four elders [the leaders of the people of God in heaven] fell down before the Lamb, each holding a harp, and with golden bowls full of incense, which are the prayers of the saints" (Rev. 5:8). Thus the saints in heaven offer to God the prayers of the saints on earth.
Angels do the same thing: "[An] angel came and stood at the altar [in heaven] with a golden censer; and he was given much incense to mingle with the prayers of all the saints upon the golden altar before the throne; and the smoke of the incense rose with the prayers of the saints from the hand of the angel before God" (Rev. 8:3-4).
Jesus himself warned us not to mess with small children because their guardian angels have guaranteed intercessory access to the Father: "See that you do not despise one of these little ones; for I tell you that in heaven their angels always see the face of my Father who is in heaven." (Matt. 18:10).
Because he is the only God-man, Jesus is the only Mediator between man and God (1 Tim. 2:5), but this in no way means we cannot or should not ask our fellow Christians to pray with us and for us (1 Tim. 2:1-4), including those Christians in heaven, who have already had their sanctification completed, for "[t]he prayer of a righteous man has great power in its effects" (Jas. 5:16).