Hey Kev,
Your best bet is to just use a theme that already has "split navigation" functionality built into it. Off the top of my head, I believe the Flex theme by OotS has this built into it (https://outofthesandbox.com/products/flex-theme).
You can browse the Shopify theme store, and test out demo versions, to find others that support this (I'm sure there's more than just the Flex theme -- which isn't even on the Shopify theme store).
Otherwise, you'd be looking at a fair amount of custom coding, to have the navigation menu within your current theme be re-done to work like this. It wouldn't be as simple as just copy-and-pasting a block of coding into the theme to make it work. The navigation would have to be completely re-coded.
The price to get the navigation completely re-coded would probably exceed the price of just purchasing a new theme that already has this functionality built in. However, if you're sure that you want to go the custom-coding route, then I'd recommend hiring someone from this page:
https://www.shopify.com/partners/directory/services/store-setup/customize-theme
I hope the above helps! 🙂
Cheers,
Stephen
