here a short conclusion how to connect two 2.4GHz receiver and the 900MHz Backup receiver to the Powerbox:
1st: Setup all receivers the same way (Framerate: 10ms, Failsafe: inactive, PPM-Setting: direct)
2nd: Set one E-Output of each receiver to UDI16, so all receivers output UDI16.
3rd: Use an Enlink 2RS plus flashed with the PPM-Firmware!
Than connect the receivers as follows:
One of the 2.4GHz receivers goes directly to RX1 of the Powerbox.
The second 2.4GHz receiver goes to the master input of the EnLink.
The 900MHz receiver is connected to the slave input of the EnLink.
The EnLink output is than connected to RX2 of the Powerbox.
Finally you need two patch leads(only plus and minus!!!) to deliver power to the receivers connected to the EnLink.
You can take the Power out of two unused outputs of the Powerbox.
So, I hope I did not overseen something...