To be honest, I don't think you can make this work if you use any kind of sessions.
Sessions are (sub)domain based, so a logged in user for tutorials.myweb.com will not be logged in on galleries.myweb.com.
If sessions are not an issue, you can set galleries.myweb.com as a server-alias for tutorials.myweb.com. Out of the box, this will bring visitors to galleries.myweb.com to the same page as tutorials.myweb.com.
From there, you can force a redirect with an .htaccess file, which should be something like this:
RewriteEngine On
Options +FollowSymlinks
RewriteBase /
RewriteCond %{HTTP_HOST} galleries.myweb.com
RewriteCond %{REQUEST_URI} !^/galleries/?
RewriteRule ^(.*)$ galleries/$1 [L]
I'm not familiar with Digital Ocean so I don't know how to set server aliasses, maybe somebody else or the DO support can help you with that.