This question already has an answer here:
Assume 2 planetoids (one of which could be Earth), similar to Earth (says similar radius, and a gravity force ranging from half to twice the gravity of Earth). Is it plausible they are tidally locked near enough so that we could "climb" (using aircrafts) from one to another (say 20 km of distance, which is reasonable I think; much lower gravity would allow also for higher buildings ^^ ).
As far as I know there are two main constraints:
Roche Limit (any planetoid closer than that limit would measure an "upward" gravity force at some surface point).
Centripetal force (any planetoid near enough would rotate around the other at so great a speed that there are points on its surface where you could measure an "upward" gravity force).
In both cases the upward gravity would make the planet disintegrate (because rocks would just float into the space).
Given the above constraints, is such a scenario plausible? The Roche limit approximate formula seems simple enough to show that is not possible (at least for planets similiar to Earth). The other "duplicated question" also take as accepted answer "no" wich is wrong because there exists binary stars systems tidally locked wich start close enough to each other to have reciprocal atmosphere exchange.