Some questions about how proper add 2 CSS file in a WordPress theme?

It seems like you’re using dependencies the opposite way of how they should be used.

In the WordPress Codex, they try to explain how it’s supposed to be used.

$deps (array) (optional)

Array of handles of any stylesheets that this
stylesheet depends on. Dependent stylesheets will be loaded before
this stylesheet.

Since your style2.css is used to make changes to style.css, it means style2.css is dependent on style.css.

This is what your code should look like:

function wpb_adding_styles() {

    wp_enqueue_style('main-css', get_template_directory_uri() . '/style.css', array(), '1.0', 'all');
    wp_enqueue_style('style2', get_template_directory_uri() . '/style2.css', array('main-css'), '1.0', 'all');
}

Hope this helps!