It's well-defined:
8. Declarators: [dcl.decl]
3) Each init-declarator in a declaration is analyzed separately as if it was in a declaration by itself.
And the note:
90) A declaration with several declarators is usually equivalent to
the corresponding sequence of declarations each with a single
declarator. That is
T D1, D2, ... Dn;
is usually equvalent to
T D1; T D2; ... T Dn;
where T is a decl-specifier-seq and each Di is an
init-declarator.
For completness (because the note says usually):
The exception occurs when a name introduced by one of the declarators
hides a type name used by the dcl-specifiers, so that when the same
dcl-specifiers are used in a subsequent declaration, they do not have
the same meaning, as in struct S { ... }; S S, T;
// declare two
instances of struct Swhich is not equivalent to
struct S { ... }; S
S; S T; // error`