I have thought of one way to do this, if I was going to do it. Without manually adjusting each map extent.
If they were individual shapefiles, within QGIS you can right click on the shapefile layer (of one) and then choose zoom to layer.
Now when you choose the map extent in the print composer this will be automatically zoomed into the specific shapefile.
Within the print composer you can then select the layer, and go to the item properties field on the right and select to lock the layer.
When this method is repeated and by zooming onto another shapefile within QGIS and then with the map extent placed, it will keep this shapefile within the print composer unaltered. Upon locking the next this method can be repeated indefinitely.
I can't think of a better way without manually zooming into the shapefiles.
Hope this helps!