The heavens opened
is a phrase used in Acts 10:11 to describe divine revelation (Acts 10:11) and it seems to have the same intention here. This description of the Son of Man standing at the right hand of God is a revelation granted by God and not an inference made by Stephen or the apostles.
56 And he said, “Behold, I see the heavens opened, and the Son of Man standing at the right hand of God.”