@mfalkvidd Hello, today I used your suggestion, but in the meanwhile the code of RelayActuator.ino has changed a little bit.
So this is what has changed and how I adjusted. This could be useful to another beginner like me.

Change:

digitalWrite(message.getSensor()-1+RELAY_PIN, message.getBool()?RELAY_ON:RELAY_OFF);

to:

if (message.getBool() == RELAY_ON) {
    digitalWrite(message.getSensor()-1+RELAY_PIN, RELAY_ON);
    wait(500);
    digitalWrite(message.getSensor()-1+RELAY_PIN, RELAY_OFF);
    }

Thank you all for your answers!
I'm not a programmer, the best I can do is to try to implement the software others created, sorry.
But it would be very nice to have a controller and both radio on a single Rpi.
If I'm not wrong, the RPi having 2 SPI chip enable, has the possibility to control those two radio. So it shold be just a software question, not a hardware one, isn't it?

Hello, I'm new to this forum.
Is it possible to have both radio active at the same time on a RPi gateway? Why did you choose to have just one at a time?
Thank you,
Andrea