Redirect certain posts to new site

I am trying to take part of a wordpress site to form a new site and would like the old pages to redirect to the new

Eg I’d like mysite1.com/item123 mysite1.com/item456 mysite1.com/item578 to redirect to mysite2.com/item123 mysite2.com/item456 mysite2.com/item578.

(those pages not listed should continue to serve from mysite1)

I believe this is done with the .htaccess file and have tried various combinations of Redirect and RewriteRule but so far without success, attempts to access the required pages produce server errors.

mysite1.com is protected by bulletproof security which amends .htaccess so I’m not sure if this affects any possible solution.