I've moved this thread to RGSS3 Script Requests. Please be sure to post your threads in the correct forum next time. Thank you.
Tsukihime has a script that lets you swap map tiles around. I've written a script that will let you draw parts of a map on a second map, and use a script call to pull that onto the current map. If I have time I'll see if I can separate it from the rest of my project and post later today.
You could do it using events, though I would advise against it. You could also just have two completely different maps, and make all your transfers go to the correct one depending on where you are in the game, but that would require having the same events on each map which could be problematic.