You're talking about tandem anchoring, and it has been discussed quite often on this forum previously. However, I generally don't recommend it, as a properly sized primary anchor
should be sufficient to the task. If it isn't your ground tackle
is undersized at best.
Part of the problem with using tandem anchors
is that it is very difficult to set the two anchors
sufficiently. If the two anchors
are the same design, the primary anchor will usually set before the secondary, because it is closer to the boat, and the load on the anchor rode
will cause it to set, and unless the connection between the two anchors is perfectly taut when you start backing down to set them, the first will set before the second gets a chance to.
Some anchors have a specific attachment point on the crown of the anchor for tandem anchoring setups. That is generally the best location to use to attach the two.
The anchors should be separated by at least a boat length of chain IMHO. Anything less than that will generally mean that the secondary anchor is trying to set in ground disturbed by the primary, reducing the holding power it can generate. This is also a big reason why a larger single anchor makes more sense than two smaller anchors.
Finally, if the tandem anchor setup is exposed to a reversing wind/current, the two anchors may foul one another, preventing the setup from resetting properly. This is far less an issue with a properly sized single anchor.