Overtime as your dreadlocks mature, or sooner if you're using the 'natural' neglect method you will find that some of your dreadlocks will start to stick together. Sometimes they will randomly latch on with a few hairs but they can start to grow together at the roots.
When they grow together at the roots you will have to rip or cut them apart otherwise all the conjoined dreadlocks will grow together and form one thicker dreadlock, this is normally referred to as a congo or congoing dreads. This effect does not happen quickly, it will look like the two or more dreads are slowly joining together more and more, but what is happening is the new hair at the root has started growing together as one. So without work and encouragement you won't be able to take two dreads and fuse them together completely, but once they've started to congo and if you don't pull them apart they will eventually turn into one fatter dread with the original thinner ones still distinguishable at the end. You can then cut off the separate thin tips if you wish.
I stopped ripping mine a few years ago so now I have about half as many individual dreads as I had to begin with, as they have paired up and congoed together.
This thick dreadlock has a single bass, but splits into 4 at the tip as it is a congo formed from 4 individual dreadlocks.
You can encourage the congo effect by banding the dreads together, but don't use rubber bands because they can get stuck in the dreads. Use a hair bobble at the tip of the 2 or more dreads you want to grow together and you should find that you will then have the two dreads growing from a single root. I would recommend you remove the hair bobble every now and then to make sure that the loose hairs haven't started to dread over it - you don't want it to get stuck!
You'll have to watch and maintain your dreads if they're already as thick as you'd like and rip accordingly. This will have to be done a lot more frequently if you're using the neglect route where you'll have to rip them a lot at the start to get them into the segment/dread sizes you'd like.