Setting up MultiSite in one main domain, that you have SSL, and then use the other domain as custom sunrise domain settings. You have to setup the .htaccess or vhost(for NGiNX) though. But both site will need to have almost same type domain or plugin. Its one way.
Other will be a bit complex one. I would setup two separate WordPress in two separate subdomain. Then from the vhost file I will point other root domain also to that same sub domain's root folder. You will need some tweaking in vhost or .htaccess to dynamically rewriting domain, and also hardcode home and home directory url from wp-config.php. Its far more complex process, might need some trial and error testing.
BTW I hope you know you could even buy a SSL certificate as low at just $10 USD, and if you use service like CloudFlare, now even in their free package they provide free SSL support. Though its not actually true SSL, but might serve purpose for normal low-end need.