For the soldering twist the wires together to form a mechanical bond between them would greatly increase the tensile strength of the connection as well as the electrical conductivity of the joint, also don't form a globule of solder on the end of your iron and bring that to the joint you will get a cold solder joint that in the conditions that you will be putting it though will cause it to fail what you should do is heat up the area where you want the joint then bring solder to now heated area and it should melt the solder without you ever bringing the solder and the iron in direct contact, once you have added enough solder to the joint for it to have formed but not so much that there is a blob remove the solder then remove the iron to form a nice shiny joint (assuming that the solder used has lead if not it will be shiny for a moment before becoming a dull silver). Finally when splicing it is best practice to avoid having all your splices at the same point in the wire since the joint will be thicker than the normal wire and having 4 or 5 at the same point causes there to be a unfitly bulge that is hard to cable manage.