Add a tilemap to the edge of a tilemap?

[Sanic]

So I've got a tilemap which a player can traverse, and at the edge of this map I'd like to load another tilemap such that a player can seamlessly travel between them as if they were one map. At the edge of that map there is another map, and so on.

I'm just wondering how to achieve this? And if it's doable without much lag... I know I can put everything into one big map but I assume that's going to cause problems... with that much info sitting in memory all at once.