SashForm sashForm = new SashForm(parent, SWT.VERTICAL);
Composite mainPart = new Composite(sashForm, SWT.NONE);
Composite second = new Composite(sashForm, SWT.NONE);
sashForm.setWeights(new int[] {1, 0}); // normally the second child should be hidden
// ...
int totalHeight = sashForm.getSize().y;
int desiredHeightOfSecond = ...;

sashForm.setWeights(new int[] {totalHeight - desiredHeightOfSecond, desiredHeightOfSecond});