How to create a differencing disk (a chained one) from a disk that is already a differencing image? I would like to have:

 W2008 (base immutable) 
  -> W2008+SQL2008 
     (differencing, with SQL installed) --- This I can do.
    -> W2008+SQL2008+SharePoint 
       (chained differencing with Sharepoint installed on top of SQL2008)

There's some info about it the manual: http://www.virtualbox.org/manual/ch05.html#diffimages

Differencing images can be chained. If another differencing image is created for a virtual disk that already has a differencing image, then it becomes a "grandchild" of the original parent. The first differencing image then becomes read-only as well, and write operations only go to the second-level differencing image. When reading from the virtual disk, VirtualBox needs to look into the second differencing image first, then into the first if the sector was not found, and then into the original image.*

I don't get it...