re Danny's prior comment: "... tandem anchoring. Attach 10 m of chain to the hole in the leading edge of your primary anchor and add the second anchor to that. Then take 13 metes of floating line and attach one end to each anchor."
You might try a longer length of floating line, such that the outer end is attached to the secondary anchor, with the inner end attached to the <anchor chain> inboard of the primary anchor, at a point between the shank of the primary and the windlass gypsy. This enables the line to be more readily accessible than if it is on the anchor when resting on it's roller. I saw this arrangement on SV Samantha, earlier this year in St Maarten; Rudi swears by this set-up for ease of use, although his set-up used 1" line between the primary and secondary anchors. I would prefer chain, for chafe protection.
I have also been wondering if a tripping line might also be beneficial for the outer/secondary anchor. More complexity I know, but if the secondary (outer) anchor really digs in, would this make it more difficult if not impossible to free the inner (primary) anchor? With just a single anchor well set, especially in mud, but also in sand, we sometimes have to be very gentle to ease the anchor out, else risk damage to the windlass. I imagine that this would become worse with an additional outboard anchor well set.
I am interested in others' experience and set-ups for the use of anchors in tandem.