Remove solder. Straighten strands. Twist both cable strands together. If unable there are 2 options:
- Remove some strands from thicker one. Then twist.
- Isolate thinner cable longer. Use thicker cable as "core" for thinner cable.
- Remove some strands from thicker one. Then twist.
- Isolate thinner cable longer. Use thicker cable as "core" for thinner cable.