Rewriting a subdomain page on one WordPress installation to a parent domain on another

If you use the main domain .htaccess file when people go onto the page you want to display on the sub domain it will change the url link to dev.mysite.com I am not sure if that is the best approach but if you want to do it that way you would have to use the main domains .htaccess to redirect to the subdomain.

Another option could be export the page’s data with a WordPress exporting plugin. A search on the website led me to this one which may be of some use. https://en-gb.wordpress.org/plugins/wp-exporter/