More than one tool could be "loaded" onto a document object at the same time but we use one at a time and switch between any others "loaded" onto the document object.
The mockups are at:
http://cit3.mak.ac.ug/~bmuhumuza/
or
http://81.199.17.43/~bmuhumuza/
Interfaces where data objects and tools are separated have been around for some time. check out
http://en.wikipedia.org/wiki/OpenDoc