There are no such graphs.
If you have directed tree-like graph - it would make no sense to go from leaves, but... from the root it will work as intended, and from the leaves - it will stop there and wait to meet the previous.
It will be wasteful, but working. In given example, the same applies - it will produce output from one side, from the second it will stop on single vertex, so it will degrade to one-directional, therefore nothing makes bidirectional search unusable.
When you cannot perform search - it does not matter whether it was bidirectional or not, it will not wirk anyway.