I can speak from an aviation standpoint and the seeming miles of hydraulic lines I've created. Although aircraft don't use double flares I've done my share elsewhere. A double flare needs to be formed on a good flaring tool or you will continually crack the flare. A quality flaring tool is bench mounted device with a very smooth faced flaring die and a set of dies that grip the line and hold it perfectly square to the flaring head. But even before you start the flare using any tool the pipe end must be burr free and smooth.
Use a tubing cutter and slowly work the tool around the line so you cut it clean and don't deform the line, such as pinching the end down from too much pressure on the cutting wheels. With the pipe cut use a deburring tool and clean up the end of the pipe. Then use some emory paper or a good set of swiss files to smooth the end of the pipe. When the pipe is ready there should be no evidence of scratch lines or knicks. This will lesson the chance of the line splitting while it is spreading. If you can perform the initial flare without the line splitting then the second inward bend will not (shouldn't) split as the tubing compresses back inward.
