I can only partially answer your questions:
You can use one device identifier (which is assigned to any of your applications) in all your applications. But, in my opinion, this is a very bad idea. What for. Because it produces fake statistics (one application specified in AdMob shows all advertised statistics of all applications). In the end, you won’t even see how to optimize your income, because you don’t know which application generates a lot of money and the other doesn’t. (for example, if one application does not lead to earnings, you can place your ad somewhere else in the application). Thus, one device identifier for each application would be a good solution.
I can not answer question 2 because I never thought about something, and I propose to reconsider my intention, because too many ads will annoy the user, and you will no longer earn, because the user, of course, will not double click because you are using two banners. He probably won’t press because it annoys him. And I'm not even sure if there are different ads showing at the same time.
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