Queues question

We have two queues with common dynamic members in both queues.
Both queues have the same weight.
How does freepbx decids from what queue it delivers calls to the members. ?
Is there a way to deliver the calls to the members based on the oldest call in the two queues ?

we are using Freepbx distro 4.211.64-7 and asterisk 11.
Changing the weight seems not to be an option because queue A is never empty and the calls in queue B will never be delivered in such case.Also making a queue C as destination for both queues A and B is not an option too because we will loose all statistics we need for the queues A and B.

some help ,please