From the mid 1930s to the late 1940s Tullio Campagnolo manufactured his Campagnolo Corsa derailleur system, which had two levers. It operated as follows: first you used the top lever to open the hub quick release, then you allowed the wheel to move forward along a toothed rear dropout to release the chain tension, then you back pedaled and shifted the chain using the lower lever to move a derailing fork, then you used the top lever to lock the quick release tight again - or something.
Marcello Gambato thought he could do better. He patented this design in which the big lever is used to swing the whole dropout forward to release the chain tension, you then back pedal and shift with the little lever and finally re-tension the chain with the big lever. It is a much cleaner and more secure design in which the wheel stays firmly attached to the dropout at all times and chain tension is managed explicitly using a lever, rather than by feeling your way, moving the back wheel in the toothed dropout using the pedals.
There is a legend that Marcello Gambato worked with Tullio Campagnolo before he left to found his own company which eventually became Gian Robert. If this legend is true, Tullio might have done well to adopt Marcello’s design.