Sometimes it is necessary to fit a couple of resistors in series because you don't have the correct value. I sometimes do it when I am trying to improve the resistor working voltage because the pd will be developed across both rather than just one of the correct value.
But how to join them?
Twist them together a bit neater than I have done in the photo?
Tin the ends and just solder them together?
When you want the job to look neat without any spikes for HV to jump off you might want to consider my method.
Obtain some 1/16 brass tubing from Hobbycraft or similar, the inner diameter is perfect for sliding the ends of your resistors into. Cut the length you desire with a dremel if you have one, if you don't , treat yourself they are invaluable.
Tin the ends of your resistors and push them into the tube, place your soldering iron on the tube and the solder will flow to make a perfect joint.