That’s a toughie! The longer you’ve had the locks, the harder it is to combine them. Firstly, wrapping them may have been too constricting for them to come together properly, but there are still a few things you can try.

If they are still loose enough that you can get your finger through them in the middle of the dread, you can make holes from the top of the dread down, and weave another dread (the one you want to combine to it) through the holes. Alternate which dread you make a hole in and which you put through the hole, and do this all the way down the dread until they are fully woven together.

If you can’t get any holes through the locks, try finding some thin, tough thread near your hair colour. Wrap the thread around the roots of the two locks (mostly undreaded hair) and tie it tightly. Do this about every 1 - 1.5 inches down the dreads so that they are secured in a few spots. If this technique works for your dreads, they will grow together after a couple months, and then just cut out the thread. The locks will be growing out together from where you tied the undreaded hair at the root. If the two dreads are too well-formed to be able to dread together where they have been tied further down, you may have to be happy on just the roots growing out together and after many years you will eventually have one whole dread.

Other than that, generally if you're four months or more in, you should just let your locks do their thing and not worry too much. They’ll work themselves out on their own. Back-combing and re-waxing on a regular basis is only necessary until the dreads are strong enough to hold together on their own and to keep the ends from unraveling too much. Honestly, don’t sweat it and have patience - they’ll end up looking great no matter what.

Here's a great tip sent to us by Knotty Niggles:

"I have recently joined a lot of my dreads together to make them appear thicker - At 1st, I did try the elastic band option, but they kept snapping and I was forever replacing them which turned out to be a right pain! - So my advice is to use a little thing called needle and thread - Be sure to have a thick needle, and some good strong thread (I used linen thread which is usually used on canvas, and I'm told it's the strongest you can get!).
Shove a piece of thread through the needle (about 20cm long) and tie it into a loop (tie it a good few times so you get a big knot, else it will just go in the hair and straight out the other end!) Wrap your dreads around each other really tightly at the root -  Thread  into your hair until it stops, and then just keep threading it around/and in and out of the dread until you run out of thread. Now you can just snap any excess thread off with a pair of nifty scissors.
To keep them nice and tight, keep wrapping them tightly, and thread every couple of inches down -At 1st, I tried using just thread at the roots and the tips, but then they are loose in the middle and you can see the gaps quite clearly. So if you want nice thick round gapless dreads, the more thread the better!
And to top it off, a nice coat of that delicious smelling Knotty Boy wax - Works a treat!"