Using a different navigation bar on a set of sub pages.

I have my main menu on all of my pages on my site.
I am experimenting with an e-commerce section for a client.
I want my navigation bar to have another heading for online shopping.
When the user clicks this, i want it to take them to a new set of sub-pages
with only the shopping navigation tabs.
I will also need a link back to the main site pages navigation bar.
Site is a duplicate test site of my clients site so is not live.

https://www.myretailstore3.trevssite.com.

I have tried to place a sub-menu on the top bar, but this not how i want to do it.
For example, when the use clicks on shop on the main navigation, it takes them to
another part of the site only with different nav bars.

Any ideas on how to achieve this?