So what I'm looking for is some plausible idea of two human planets, orbiting each other closely, that despite having regressed to a medieval homage, were capable in this old era of powering solar sails through vague recordings and derelict technology, leaving their atmospheres to raid/settle/explore the other planet, hopefully without the need or explanation of how they maintained thousands of colonists/warriors with swords and shields en route, round-trips.
I've read and vaguely remember about the concepts of gravitational dissonances that can cause areas somewhere to be slower or faster when it comes to passage of time. Not sure what that is called. Yet, I also remember something else about if we were able to travel at 1g, etc in space, we'd reach places far faster like Alpha Centauri. That's what I had in mind so far.
The idea so far, is that a 3rd intelligent planet seemingly disappeared after an apocalypse/conflict that rendered both planets to food and climate catastrophe. Both the current home-worlds recovered completely, but without keeping most of their old tech, lore, and populations in the billions intact.
Somehow the 3rd planet after its flight, left behind a gravitational field for several millenniums which until recently dissipated, almost allowed immediate transfer of primitive starships from one atmosphere to another. This lore isn't necessary for the present scene, but it is apart of the foundation of why there are so many issues and population transfers between both planets that ebb and flow in technological development.
I could imagine some sort of Dune/40KMechanicus priesthood on both worlds facilitating the transports and their maintenance with the political consequences of granting religion that stranglehold, not my cup of tea, for both at least. Its the explanation and **sounding scientific ** I want for a French men-at-arms for example, that is able to escape Earth's atmosphere and enter Moon/Mar's atmospheres (Distance) only needing some 17th century submarine to go back and forth without breathing apparatuses, or nuclear engines, maybe some sort of gravitational slingshot that overcomes NASA-tech to survive in space. I know it can't be realistic, but some theories that could support it?