There are several ways to combine dreadlocks.
Our preferred way is to simply crochet two dreadlocks together at the root. Or for the full length, we would crochet extra hair down the entire length of those two dreadlocks to hold them together, however this is a very tedious process.
The easiest way is to take two dreadlocks and group them together at the root and put a rubber band or tie string around them to hold them together. As they grow out the roots will grow together effectively combing two dreads into one. Just understand that the length of the dreadlocks will not be combined with this approach and using string and rubber bands can cause problems if done incorrectly. We have a post on rubber bands which you access here.