Planning out my next vacation, I found out that the cheapest flight from my city (A) to my destination (C) happens to have a two-hour layover in an airport near one of my closest friends (B). By searching for the same flights earlier the next day, I found out that the entire trip can be bought for the same price the next day, with the friend to destination (B-C) leg a few hours earlier.
What I'm trying to do is mix and match the flight legs from the identically priced two days, so A-B happens on the first day and B-C happens on the second day, giving me 21 hours with my friend. But, whenever I try searching for the flights separately, or as a multi-city trip, or using a number of different flight booking sites, the price seems to jump from around $600 round trip to around $1100 round trip. Is there an easy way of extending this layover from 2 to 21 hours at the same price, given I have the flight numbers in hand?
P.S. The flights are all KLM/Air France partnered flights, and the flights are indeed international.