There is no set rule for when a double wick should be used. However, a general guideline is that candles with a diameter of 4" or larger may need a double wick (or triple if preferred). The reason for using a double wick in larger diameter candles is to achieve a full burn pool, which can be challenging with only a single wick. There are large wicks designed for wider diameter candles. However, we have found that larger wicks are more prone to mushrooming, soot, and smoke. For this reason, a double wick may be a better alternative.
The decision on when to use a double wick is more of a personal choice. We have known candle makers to use double wicks in smaller diameter candles and others that prefer a single wick in a larger diameter candle. Testing is the only way to determine which wick choice (single or double wicking) is best for a particular application and will achieve optimal burning characteristics.
Double wicking a candle is really not difficult. The challenging part is finding the right wick size to use for a double wick. As a general rule of thumb on how to select the right wick size when using double wicks, find a wick that is designed for half of the candle diameter and start testing with two of that size. For example, if making a 5" diameter candle that will have double wicks, you may begin testing with a wick size that is designed for a 2.5" diameter candle. Be sure they are centered, spaced out about 1" apart from each other, and not too close to the side of the glass if using containers. Placing the wicks close to the sides can cause the glass to get too hot. Please remember that testing is the only way to determine the best wick size for a particular candle recipe.