Multsite testing at the same time I use the website

I have a non WPMUDEV theme working in Portuguese, but I want to setup a new Multsite theme, from .

O bought a new URL that is pointing to the old website. In the new domain I want to set up, make all the tests needed before making it available for people to get in.

The problem is that the new doin is pointing to the old one and when I try to enter the new domain to work in into to work with the WPMUDEV theme (Blogs MU) I can not get in there, for I am directed to the old website.

How can I solve this problem?