There are various criteria to decide how to schedule data on different cells of an eNodeB. I have listed below only some of them:
1. Eligibility of a UE to be scheduled on a particular cell. for 2xCA, only CA capable UEs can be scheduled on both cells. Non-CA UEs need to be scheduled on their respective (P)Cell.
2. Whether a particular QCI can be scheduled on a particular cell or not. Generally, VoLTE traffic is scheduled only on PCell.
3. If the channel conditions that a UE sees on a particular cell is very good, that cell may be given preference over the other.
4. Depending on the cell bandwidth, more data maybe scheduled on cell having higher bandwidth.
5. For load balancing, more data may be scheduled on a cell that is less loaded ( PRB utilization is less)