Linking blender files to each other

Hi, I want to link two blend files to each other, for example:

I have two .blend files : abc.blend & def.blend

now lets say abc.blend is the overall game world, while def.blend is the interior of a building we just entered.

So how do I link them is such a way that when I create a runtime file. Runtime file has the linking instructions.

Maybe the first thing would have been to actually post your game engine support question in the game engine support forum rather than in the News & Discussion forum.