45 degree inverted flare unions usually made from brass are perfectly legal, and fine to use to splice 2 brake lines together. As has been already mentioned many vehicles, especially trucks use them from day one.
Compression unions are not legal and aren't meant to handle the high pressures encountered in a typical brake system. Especially in an emergency situation when you really need the system to work, they can fail at a crucial time. The ferules tend to work harden and split or come loose from vibration. Compression fittings are meant for lower pressure stationary systems without the exposure to constant vibration. I.E. water or gas lines in a home. This is also where single flares belong, not in a 3000 PSI brake application as they tend to split and crack under severe pressure and vibration.
I recommend Nicopp lines for brake system repair, and fabrication. It's extremely easy to get good double flares with a hand held flaring tool and is also very malleable for forming bends, usually no bending tool is required. Made from a nickel, and copper alloy it will outlast any steel lines and doesn't have the associated headaches that stainless steel has from being hard to flare, and bend.
If you're double flaring on your own cars use a little bit of lube on the flaring tool forming cones to make them easier to expand. Just a little lube makes the forming easier, and less likely to come out misshapen.
Premium car makers are now using Nicopp lines to stave off corrosion, and premature failure of the hydraulic portion of the brake systems.
Good luck on restoring your brake system tubing, it's never a fun job.........
