Not sure if you ever got an answer to this but I was searching for something else and come across your question. Are you using oauth? I didn’t think you called device handlers through REST you are actually calling the device. It associates the MAC address from the client (your RPi) with the device.deviceNetworkId which is how ST things knows which device is associated with the incoming request which then the devicehandler gets executed based on the device it’s associated with. So I don’t see how you’d set 2 device.devicesNetworkIds to the same MAC address and even if you could how would ST know which device’s devicehandler to execute?
Look at oauth and use a SmartApp instead and that will allow you to call whichever SmartApp you want to from 1 RPi. Sorry if you already figured this out, just thought I’d throw my 2 cents in when I found this. You can look at this thread for an example of calling oauth from an esp8266. The thread is deprecated but the information is still valid.