And when the day of Pentecost was fully come, they were all with one accord in one place.
Note 1 at Ac 2:1: Saying the day was "fully come" means the sun was completely risen.
Note 2 at Ac 2:1: One of the big reasons that the power of God is not more manifest in the church today is because we are not of one accord and in one place (see note 4 at Ac 1:14).
Of course, it is not possible that all the Christians worldwide can be together at the same time in the same place, but that is possible on the local level. The fact that churches are built right across the street from each other and rent the same spaces in shopping centers or office buildings is just a token of how disunited the body of Christ is today.
Unity among believers is not a prerequisite of revival, nor is it merely a result of revival. It is an integral part of revival, without which the great outpouring of God's Spirit that all believers long for will not come.