On page 121 in Ned Schwing's book "The Browning Superposed" The double trigger 20 ga. Superposed is discussed. the gun was marketed by FN and sold mostly in Europe (not in the US). Guns were marked FN and not Browning.
I have seen a couple and I have shot one that belongs to a friend, his story is, "a relative brought the gun back to the US while in the service".
Yup; I have tried to buy the gun and he has agreed to have his wife sell me the gun if I out live him. (he is younger than I)... Sooooooooo It looks like a dead end for me...
David