If you're planning to use them for their intended purpose, forget soldering and go straight to welding.
But really, these things are not expensive. Get some tandem length shifter cables if that's what you're dealing with. Or get some cable couplers, like the DaVinci or Jtek to lengthen them even further.
S&S: DaVinci Cable Splitters — TI CYCLES FABRICATION
Shift and Brake Cable Splitters - Co-Motion Cycles
Jtek Cable Splitter (jtekengineering.com)