Redirect Multisite site to another one

My client has a multi site install. The are using the domain mapping plugin

I’ve merged several of the sites together using export/import now I want to setup a redirect from the old domains to the root site

Here is an example to explain what I need.

I’ve exported domaintwo.com and imported it to the root site at domainone.com

Now I want to setup a 301 redirect from domaintwo.com to domainone.com

I;ve tried various .htaccess setups, redirection plugins but I cannot get it to work, any pointers would be appreciated.