Let's say you have 2 stacks of Noblegarden Chocolate
1 stack with 7 days duration
1 stack with 2 days duration
Split both stacks into 10 x chocolates each.
Then you'll have
1 stack with 10x chocolates with 7 days duration
1 stack with 10x chocolates with 7 days duration
1 stack with 10x chocolates with 2 days duration
1 stack with 10x chocolates with 2 days duration
Put one of the 2 days duration stacks, into the 7 days duration.
Repeat
Voila! You have all chocolates @ 7 days duration.