Tell us about yourself!
You didn’t make the magnet layout clear when you doubled them up. Did you rotate the upper magnet 90°
I'm an old plumber...in biz for over 40 years. Make sure there is an opening in the system, a valve for example. When heating anything, pressure will increase. If there isn't ant way to relieve that pressure, it will blow out through the joint you are soldering, especially on larger pipes.