IMHO it might be that the Powerbox and the Cortex are doing partly the same job. That is taking the serial S.Bus signal and converting it to PWM (standard servo outputs).

My guess would be to bypass the PowerBox on the Five channels that the Cortex is controlling.

Use a Y-Connector out of the Rx S.Bus Port. One going to the Power box S.Bus input and the other going to the Cortex S.Bus input. Then plug the 5 Servos you want controlled by the gyros into outputs A-E on the Cortex. All the other servos would go to the PowerBox outputs.

You might run an additional power wire, a male-male servo wire power only (clip the white wire, red and black only) from an unused output on the PowerBox to an unused plug on the Gyro. Not necessary but will help bring a little extra current to the gyro running 5 servos.

Hope this helps, good luck!