One of the most remarkable instruments is the tripple flageolet, with three playable pipes. Only a few are in existence.
Double recorders and flageolets
Several historical double recorders and double flageolets are still in existence, as well as echo-flutes (these are not to be played at the same time, but are two recorders often tied together to be played shortly after one another to imply the echo otherwise often difficult to achieve on recorders, for example for BB4).
Michiel Parent (Amsterdam 1663 – Amsterdam 1710)
More info to follow
A picture of a double recorder by Michiel Parent, with thanks to Jan Bouterse: