The twins were of the dicephalus variety, joined from the sixth rib down, with four arms and two legs. They had separate hearts, lungs, and stomachs but shared reproductive organs. Giovanni, the left twin, was intelligent, talkative, and an excellent artist, whereas his brother possessed little or no artistic ability and was quiet and introverted. Giacomo was prone to tantrums and would sometimes kick his brother's drawings off their lap. Giovanni liked beer, while Giacomo preferred plain water. They ate, slept, and became sick at different times. Because so much of their time was devoted to travel and exhibition, they never learned to walk and instead scuttled about on all six limbs or were transported in a wheelchair. That their inability to walk was due to the fact that each controlled one leg is completely false - several sets of similarly joined twins have mastered walking.
The Tocci brothers disappeared from the public eye in the 1890s, returning to their native country and allegedly purchasing a secluded, high-walled home to keep gawkers at bay. What became of them afterwards is unknown. Some sources claim they died in 1912, while others cite this year as the date of their joint marriage to a pair of sisters. Martin Monestier's book Human Oddities (1978, translated from French in 1987) was the first to provide the unlikely date of 1940 for the brothers' death. Because the Toccis allegedly married sisters and would have lived 63 years if they died in 1940, I believe it is very possible that Monestier (who made other errors) confused them with Chang and Eng Bunker. In any event, it seems improbable that Giovanni Batista and Giacomo would have lived so long in an era predating advanced surgical techniques. Dicephalus twins often suffer from extreme scoliosis that compresses the lungs and impedes breathing, and the surgery required to correct this condition had not been developed in the first half of the 20th century.
Above: Replica of ca. 1890 carte de visite by Obermuller and Kern of New York City. Author's collection. The original card belongs to Dr. Jan Bondeson, author of The Two-Headed Boy, the definitive source on the Tocci twins.