If you're using the same tilesets, you can open 2 sessions of RMXP, right-click on a map in the map-tree area, and select "Copy", then go the other game, and "Paste".
If you copy the file (Map004.rxdata) to the new 'data' folder, you also need to create a slot for it in the map-tree of the new game, save the game, close it, then copy the map file over, then reopen the game.
You may also need to rename the map, unless it has the same number as the placeholder.
Be Well