Simply build the two index pages at the required size(s) using pre responsive design methods, the best type to use for both is what was known as 'fluid or liquid' design, (google both).
You will also require a script, (php and/or js) to detect the viewport size, of which there are many, (again google the subject) and deliver the appropriate page.
It should be said, that just like Muse sites that used this approach, it will become more and more difficult to get the correct site to to load on the required device. This is because both desktop and mobile browsers are constantly evolving, and a mobile device, (such as iPhone x) may be identified as a small desktop monitor, (one of the reasons why Muse failed more and more).