The second man could not answer after knowing the Sum total of 3 numbers and also their Product (his age).
This means that there exists two sets of numbers having two common = meaning 1 set of twins.
all possible combinations are;
1,1,11 = 11
2,2.9 = 36
3,3,7 = 42
4,4,5 = 40
5,5,3 =75
6,6,1=36
Even with this knowledge he could not answer because possible combination can be either
1) 2,2,9 = 36
OR
2) 6,6,1 = 36
Once the first man said "my oldest son weighs 91 pounds"
the second man could conclude that the correct combination is 2,2,9
OR say oldest Son of age 9 and then a set of twin of 2 years of age.