First of all, I will treat your category as the same category, because if you have different categories, you can definitely register successfully! But you should be clear that you are talking about a well-known trademark, not a well-known trademark. If it is a well-known trademark, even in different categories, it is not allowed, because it is cross-class protection!

Then it depends on whether the meaning of A.B has completely changed after synthesis. If it is directly obvious that it is A and B, it will often be rejected in the preliminary examination of the Trademark Office! (It's no problem to get the acceptance)

Not to mention the possibility of being challenged by company A or company B during the announcement period!