One Address - Two Domains

I've read the help links, but I cannot get my brain wrapped around things and I'm looking for someone more skilled than me to help me:

I have one website; two businesses - each with their unique domains. I'm trying to get certain pages linked to one of the domains and certain pages to direct link to the other - is this possible? If yes, can someone please dumb it down for me because what is listed on the help links DOES NOT COMPUTE to me. 

I'm self taught; not a developer so I'm super confused because the words aren't making sense. 

The easiest way, since a Squarespace-hosted domain cannot be directed to a particular page unless you have the /page name on the domain, is to build a landing page with two action keys, one button goes to one business and one to another. The landing page becomes the first page for both domains. Here is a video guide to making a landing page.  that: https://insidethesquare.co/tiny/landing-page

If you want to control the menus for each subsection of your site ( i.e. each subsection has its own menus) check out the following for a starting point.