How do you have several sites coming into the one hosting space, and have one with different robots.txt file. Correct usage of the .htaccess file enables multiple robots files.

Logic - if accessing via domainb.com/robots.txt, then show the robotsfff.txt file, otherwise show the default robots.txt file.

.htaccess file includes:

RewriteCond %{HTTP_HOST} domainb.com
RewriteRule robots.txt robotsfff.txt [L]

Then the robotsfff.txt file containing say a total exclusion:

User-agent: * Disallow: //

this means that the domainb.com/robots.txt shows as User-agent: * Disallow: //

And no need to redirect the access for the robotsfff.txt file. You could do a 301 redirect back to the robots.txt file, not worth the effort.