Even if the apostles were praying to Jesus... His church had not yet been established and He would not yet be the Mediator and High Priest.
In the passage of Acts 1:24 we also see the use of the Greek "kardiognōstēs" which means "knower of the hearts". While this power is of Jesus (John 2:24-25; 6:64; Rev. 2:18)... it is also the power of God (1Sam. 16:7; 1 Chr. 28:9; Ps. 139:1,23; Jer. 17:10). The only other use of the Greek "kardiognōstēs" in the New Testament is in Acts 15:8,
where it is applied to the Father.
The first recorded prayer
after the establishment of the church was addressed to the Father as "Lord" (Acts 4:24,29).
It is important to note that just a few weeks prior to Acts 1:24, Jesus instructed the apostles to pray to the Father, not Himself (John 15:16; 16:23-24).
When Jesus taught the apostles to pray He instructed them to "pray to your Father" (Matthew 6:6).
Below is a link to a very in depth study on praying to Jesus... MS Word document. Several of us brethren got together and studied this for 7-8 hours over a few nights. The brother that actually wrote it, studied it for much longer.
Can a Christian pray to Christ?