If I have 2 pieces of tekhelet string, can I tie them together and use it as one?
If you are starting out (i.e., not fixing a torn string that became less than “kdei aniva”) you can tie strings together to get the desired initial length. We do not advise this because if your knot comes out, your tzitzit will become invalid. This is particularly true if the point of the knot is at the top by the hole before the actual gdil (knotted section). Furthermore, if the knot does come out, you will have to untie them completely and retie them because the halacha of tzitzit requires them to be made in the usual way (min ha’asuy), and it is not usual to knot strings together after they are tied on to the garment.
– Mois Navon