@Satsuma XD this almost great but ultimately awful sci-fi movie Upside Down where two planets almost touch so people on one always are upside down to each other but only the gravity from your own planet affects you and the matter from each is incompatible, like less violent versions of matter and antimatter. if you manage to get indoors together on one world someone's always walking on the ceiling, and you can weigh yourself down with rocks from the other side and flip but then your rocks will catch fire after an hour or so

so of course two people figure out how to make a baby which will definitely be fine