What is a biplane propeller and how efficient is it?
It is essentially two propellers stack on top of each other. You can see it in this picture of a Lazair.



As for why they were chosen for the Lazair, 


  Many have asked why Ultraflight opted for double propellers stacked in
  a biplane fashion. This is a very unique setup and draws lots of
  attention. Apparently when Ultraflight changed over to the Rotax 185
  they tried several different propellers. The problem was the 185’s had
  a woodruff key slot in the crankshaft and would snap the crank
  whenever the engine coughed or kicked back on start up. This was quite
  a serious problem. After much deliberation it was determined that the
  rotational inertia of the propellers was too much for the crank to
  handle.
  
  Dale could not locate a two bladed prop light enough to fit the need.
  However, since they had been producing a lot of the early lazairs with
  Pioneer engines fitted with a single plastic prop, he had a ton left
  over that fit the pioneers.  He found that these could be “doubled up”
  and help to produce the extra thrust that the Rotax 185 was capable of
  producing over the 5.5 hp Pioneer engines.


It appears to be a quick way to gain more thrust from the props without increasing prop mass too much.