What does C++ language definition say about the extent of the static keyword?

According to the C++ 17 Standard (10 Declarations)

2 A simple-declaration or nodeclspec-function-declaration of the form

attribute-specifier-seqopt decl-specifier-seqopt init-declarator-listopt ;

And (10.1 Specifiers):

1 The specifiers that can be used in a declaration are

decl-specifier:
    storage-class-specifier
    ...

So in this declaration

static int s_One, s_Two;

the decl-specifier-seq contains two decl-specifiers, static (storage class specifier) and int. Thus the storage class specifier static describes the both variables in the init-declarator-list s_One and s_Two.

Yes, it applies to every name in that declaration:

[dcl.stc]/1: [..] At most one storage-class-specifier shall appear in a given decl-specifier-seq [..] The storage-class-specifier applies to the name declared by each init-declarator in the list [..]