Lets say each cord is 10 units long and for whatever reason the burn times for each unit are 1,2,3,4,5,6,7,8,9,15 = 60 (in Minuets), folding is half will not get you 30 sec on each side. If you light both sides they meet 1/4 the way through the 8th unit. If you have two of the cords and you light three ends the one with both burning will be gone in 30 mins the other, depending on what end you light will have 7 1/4 units left or 2 3/4 units, either way they will = 30 mins. If you light the remaining end it will take 15 mins to burn out so the total burn time is 45 mins.
Not sure why any one would manufacture detonation cord like this :-). It there a real life application to this puzzle?
My two cents!
(Really just two coins not sure if they are both worth a cent but for whatever reason I know they = 0.02 of a dollar)