Here is an additional suggestion, Stagger the connections.
By that I mean keep the splices an two to three inches apart so if they become disconnected there is little chance of them shorting.
This is about the best I can do to illustrate what I mean. This represents two parallel wires, in your case, one red, one black.
O-------------------------[splice]------------------O
O-------[splice]------------------------------------O
The splices are not side by side, they are offset.
You can try buying one of those Crimping Tool Kits. They are very commonly available from many sources, just walk in and buy a kit, and they come with Butt Connectors. Simply try to splice two scrap piece of cable and see if it works for you.
You will only be out £10 to £15, and if nothing else, you will still have the tool and the terminals.
The tool is also good for Stripping wire. And though you don't need the feature, the tool can CUT common Machine Screw down to a desired size.
Draper 5 Way Crimping Tool And Terminal Kit : Tooled-Up.com
Myself, I would solder and use Heat Shrink tubing, but I have all the equipment and experience to do that.
Steve/bluewizard