Map 2 domain names onto one site

I have a multisite setup and wish to map 2 domain names onto one of the sites.

Example, original site location:

First mapped domain

Second domain to be mapped

I have done this mapping for half a dozen sites, and believe myself have done the DNS stuff correctly.

My problem now is, I found no way the "Domain Mapping" plugin let me input the second domain name.

Any help?