Two URLs for same blog site?

I’m trying to do a simple Apache RewriteRule in a multisite installation and for some reason, it’s having no effect at all. For example, I’d like https://example.com/yourblog/.* to redirect to https://example.com/myblog/.*. So I added this to .htaccess:

RewriteRule ^yourblog/(.*) myblog/$1

The rule has no effect at all when I hit https://example.com/yourblog/. I get a 404 error rendered in a WordPress page, as if the rule weren’t there. So something (a 404 handler?) is overriding my URL before the RewriteRule can take effect.

Even a silly rule like this one has no effect:

RewriteRule ^yahoo https://www.yahoo.com [R]

What’s the right way to give a blog site multiple URLs?

The current .htaccess file is pretty standard.

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

Thanks for any help.