Calling two namespaces for formbuilder and controller in d8
A class can only be in one namespace. There's never a scenario when a class being in two namespaces could be useful, and the language doesn't allow it.

I have no idea what you're trying to achieve, there's no mention of that in your question, so again I'll just reiterate (3rd time now!) that you should learn what namespaces are, what autoloading is, what PSR4 is, and how to use them. Otherwise you're just going to spend your time banging your head against these trivial PHP language issues, and get very little done.