Multi-Site & Multi-Network. Use the same Domain Name?

Hi.

I have installed the 'Multi-Network Plugin'; and undertaken the required "comment out" action in the WP Config.php file!

In your WPMU Post:-

"How to Create a Network of WordPress Multisite Networks

By
Jenni McKinnon
October 4, 2015" the following guidance is given :-

"Click Add New to create a new Multisite. Next, enter your desired Multisite Network Name. In the Domain field, you can type the domain you wish to use for your Multisite".

"You have a few options here. You can enter your multi-network’s domain, then under Path, type in a sub-directory name you would like to use."

"Your URL should end up looking similar to http://www.your-multi-network.com/Multisite1".

"Alternatively, you can enter a new domain that you own with or without a path. Just make sure the new domain is pointed to your multi-network’s site path".

However, I am unable to create a 'New Network', using my 'Main Multi-Site' 'Domain Name'.

Yet I can create a 'New Network' if I use a different 'Domain Name'!!

Have I misunderstood ( misinterpreted) the above guidance, please?

Thanking you in advance.

Regards

Johnv