Single wp-admin for diffrent domain site in wordpress

I have two different domain

1)firstsite.com ……….. Domain register http://godaddy.com/ and hosting justhost

2)secondsite.com ………. Domain register http://godaddy.com/ and hosting justhost

Both site have diffrent Cpanel,Ftp,Database

i have installed wordpress both site root directory

Task: Make single wp-admin for both site(mean to say operate both site with single admin panel)

i have follow those step do it on firstsite.com site

1)First create multisite network setup with help below link

Create A Network

2)For Domain mapping use below link

WordPress MU Domain Mapping

but that setup is successfully working for sub domain not for separate domain(i can include other site like sub.firstsite.com and there is no option for separate domain secondsite.com)

so plz guide me wht i m doing is that correct way or its not possible.