After some research and some test I came to following solution:

We will need a Master, which displays the current state of each target. Also this Master holds the target software. For some reason (price and size) i choose a Raspberry Pi 3 with Wifi and Bluetooth.

The Slave are based on an Atmega328PB which was brand new that days. The reason is that I need two serial Interfaces. One for the communication with the Master and on for the targets. Because our target have AVR Microcontrollers too we will have no problems even without level shifting.

The communication between the master and the slaves is a UART based bus. To prevent any weird behavior the slaves are not aloud to send anything as long they are not getting a specific command. Each slave has an individual address. The master can choose each slave individually or broadcast to all.

The slaves can communicate with the targets normal via UART. Also, each slave measure the current of his target, enable and disable the power, can measure some voltages. For USB test, there is a USB Host chip on each slave.