[Resolved] Split: email notifications.

[Author]

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[Waqar]

You can get the admin email set at WP Admin -> Settings -> General, by using the "get_bloginfo":
https://developer.wordpress.org/reference/functions/get_bloginfo/

$admin_email = get_bloginfo( 'admin_email' );

And to get the current "vakexpert" post's author email, you'll need to use "get_post_field" and "get_the_author_meta" functions:
https://developer.wordpress.org/reference/functions/get_post_field/
https://developer.wordpress.org/reference/functions/get_the_author_meta/

$vakexpert_author_id = get_post_field( 'post_author', $parent_id );
$vakexpert_author_email    = get_the_author_meta( 'user_email', $vakexpert_author_id );

And these can be combined in the fine code snippet, by replacing the line:

$to = 'abc@xyz.com';

With:

$admin_email = get_bloginfo( 'admin_email' );

$vakexpert_author_id = get_post_field( 'post_author', $parent_id );
$vakexpert_author_email    = get_the_author_meta( 'user_email', $vakexpert_author_id );

$to = $admin_email.','.$vakexpert_author_email;

[matthiasV-2]

My issue is resolved now. Thank you very much Waqar!