I'm not sure if you're suggesting that you're getting a knot on one string or if your strings are knotting together.
If it's the latter, just make sure that you isolate the two threads that you're working on in the moment and push everything else off to the side. In time, you'll get quicker and you'll find yourself tossing threads aside and grabbing the next two with ease.
And, either way, I suggest making sure the thread does not get twisted on itself by running your fingers between your threads. Almost like brushing your hair - make a brush with your fingers and brush out the twists.
If all that fails, shorter threads are easier to work with than longer threads. A bit of trial and error will help you gauge how short you can go for the project at hand.
For me, the key is staying organized and keeping my threads in order.