Compression fittings have no place in a brake system, they just aren't safe. There's no reason to use these as a fix for splicing brake lines, as double flaring a brake line is really an easy process. Most compression fittings are only rated 300-500 PSI depending on size and quality. Your standard braking system will easily see over 1000 PSI in many situations. These are your brakes, there's no excuse for shortcuts. Single flares can come apart as well under this kind of pressure, thats why double flaring brake line is a must.
Double flaring tools can be rented or bought from most parts stores. You'll also need a tubing cutter. The line that you are flaring must be cut perfectly straight, so using a hacksaw is out of the question.
You want to start by inserting the brake line through the flaring tool to the first step of the double flaring adapter.
Tighten the wing nut closest to the line first, then tighten the other wing nut.
Then insert the double flare adapter into the end of the line.
Now crank down on the flaring tool until the double flare adapter is pushed all the way down.
If you did it right, you should have what looks like a bubble flare at this point:
Now remove the double flare adapter, and use just the flaring tool to finish the double flare:
Now at this point you should have a perfect double flare: