Unless space can be saved, a one-cell contraction or two one-cell contractions are usually preferred over a two-cell contraction. Examples:
Apartheid. The two-cell contraction for part is used in apartheid to indicate proper pronunciation, even though it uses more space.
Haddock. The dd contraction is preferred over the had contraction in haddock. However, when the word is divided between braille lines, the had contraction is used.