I looked at my copy and there is a little there. First, Schwing pictures a copy of the 1931 patent for the twin single trigger, and has some discussion of the trials and tribulations that Val Browning and FN went through in working out a trigger mechanism. He characterizes the Twin Single as "...simple yet effective, but not very popular". He references a letter he had from Val Browning that the twin single was an attempt to deal with a desire by American shooters for a pistol grip which did not allow easily moving between two triggers. He indicates the twin single trigger was an additional $20 over standard double triggers, but when the original Lightning was brought out about 1937 as a sort of "economy model" the twin single trigger was standard on it. Finally he indicates that when the design for the second version of the selective single trigger was perfected in the late 1930's the twin single and other triggers (double, single non selective)were all discontinued. He does not say exactly when this occurred, but he seems to say it was at some point before production was halted by the war. (Standard double triggers were always available on guns sold directly by FN outside North America.)
Schwing had access to both FN and Val Browning but treated the early superposed basics pretty lightly. He was clearly more interested in engraving and high grade guns.
