Whatsapp backup in Google Drive Does Whatsapp backup in Google Drive differentiate with one Gmail account from 2 different devices with different Whatsapp account?
To be honest, it is quite straight forward how Google Drive back up work with WhatsApp. 

For a WhatsApp account, which is tied to your phone number, lets say, you used abc@gmail.com as the Gmail Account to Store WhatsApp back up. 

So, all your back up is now with abc@gmail.com. You should remember this Gmail Account and use this Gmail account to restore the back up of your WhatsApp Account.

Now, for Another WhatsApp account, which is tied to another number, you can use the same Gmail Account, in theory, which is abc@gmail.com, and you can restore the back up of this WhatsApp account using the abc@gmail.com

It really does not matter, which phone from where you are accessing the WhatsApp, as A backup file does not exist on the Google Drive account or locally on the phone.

You can read about the WhatsApp back up on Google Drive on Official FAQ page of WhatsApp.