Multiple definition class in arduino
If you want Fooa() to be defined in a header that can be included in more than one .cpp file, you would need to declare it as an inline function like so:
#ifndef Foo_H
#define Foo_H

class Foo
{
    public:
    Foo();

    public:
    inline bool Fooa();
    bool Foob();
};
#endif

Alternately, you can provide the function definition directly inside class definition; this also makes the function inline despite not requiring the inline keyword.
Or if you want to keep Fooa as a non-inline function, you'd need to move the definition out of the header and into a .cpp file as you did with Foob().
Making a function inline has a second effect beyond implying that you want a function's code to be generated at the call site; it informs the compiler (or more accurately the linker) to expect to see multiple definitions (that are required to be identical) of the code for the function.  Without that advanced noticed, you will get the multiple definition of error when a function definition is inserted into multiple .cpp file by way of an #include.
As a side note: #include is an automated form of copy-and-paste.  It logically plants the contents of another file (and all of its reachable #include uses transitively) into place where the #include directive appeared.  It does not behave the same way that many similar looking directives in other languages do.  E.g. #include does not behave like the import keyword does in say Java or python for example.