I spent quite a bit of time searching for a way to have a SashForm update as I was dragging the sash; after many dead ends, I finally stumbled upon the SWT.SMOOTH style (edit: also requires one to setMaximizedControl as well). Why is this not listed in the SWT Designer for the SashForm under style?
second edit:
After playing around with it some more, you don't need to set the maximized control. The test i did was to create a class that extended the ViewPart, used the designer to add a sashform, added a tree and a table, clicked over to the source changed the SWT.NONE to SWT.SMOOTH (at this point, the preview still is the vertical line on a drag), then switch back to the designer, then open the preview.