It is a good idea to use two anchors when anchoring in tidal areas or when there is a front approaching. In both cases the boat will be swinging on its anchor to a new direction. And when that happens the anchor will have to break loose. Some anchors can reset quickly and safely, but many do not. And even the good ones may cause some problems.
There are two ways to use double anchors. If the change of direction is not too radical, you can place two anchors ahead of the boat. It is important that you lay the anchors out at an angle.., not in line.
To do so, lower the anchor and allow the boat to drift back slowly applying more and more resistance on the cleat until you have the minimum 7:1 anchor rode (length of anchor rode let out is seven times the depth of the water) and then some. Now, go forward at a 45-degree angle to the present anchor until you are abeam of it, lower the second anchor and drift back to where the anchor is set at 7:1 ratio. Secure them both and you are safe.
For radical changes (i.e., a total reversal of tide) you may need to set at 180-degrees from the boat. To do so lower the windward anchor, then let out enough anchor rode to more than double the 7:1 ratio. Then lower the downwind anchor. Now go windward again until you are directly between the two anchors and secure them both. This will allow the boat to swing in both directions.Information courtesy of The American Boating Association updated: 4/1/2008