Brake line flaring tool that actually double flares steel line?

The steel tube is probably slipping in the jaws. Try replacing the wing nuts with hex nuts so you can tighten with a box wrench. Chuck the flare tool in a vise for better control. Tighten the nut closest to the tube first since the one further away has more leverage. "Should work". BTW, if you ever need to make a bubble-flare, as on most cars ~2000+, you can use a normal dbl-flare tool (youtubes). Later "metric" tubing is commonly still 3/16" (4.75 mm), only the tube nuts vary (metric thread). For the same tubing size, tube nuts come in various thread sizes, both SAE and metric. Sometimes, that is so you don't mix up connections, like at a Master Cylinder.