I use a two anchor rig (100# Danforths) strung between 84 ft of very heavy (1") chain. The 40 ft top chain (1/2 in) is attached at the mid-point of this bridle. At the surface there's a swivel, ball and pendants. All of this gear is sized for a 25 ton, beamy sailboat w/ a big mast (lots of windage) moored in 25 ft +/- water depth at high tide.
Several years ago the harbor master in my town was asking people installing new moorings to use two (or three) anchor rigs. I think he was trying not to have the harbor bottom littered any more than it was with concrete blocks. I went with his suggestion, but others resisted. Tradition dies hard. He's since backed off the anchor request on new moorings.
If you're going to use anchors, my view is that a two anchor rig is preferable for two reasons: one it allows you have reasonable scope with a smaller swinging circle (a big advantage in most harbors), and two you'll never have to worry about an anchor pulling out and having to reset with a big wind shift.
One thing you need to consider in using a two anchor rig is getting both anchors set well with the bridle chain fairly tight between them. Because of the weight of my gear I hire one of the local fishermen to help with their big engines, booms, etc. We start by getting the anchors shackled to the bottom bridle chain with the top chain w/ bouy attached in the right place. We then lower one anchor in position to set and hold against the prevailing wind. With the first anchor on the bottom we slowly feed the bottom chain over the side letting the wind move the fishing boat down wind as it goes. When the mid point of the bridle is reached the top chain and bouy go overboard and we continue letting out the bottom chain. As we approach the end with the second anchor we tie a heavy line to the second anchor and then lower it over the side. The line is tied so the flukes are free and will set without the line fouling them. Before the second anchor hits the bottom we start to pull against the first anchor stretching the bridle as tight as possible and beginning to set the first anchor. When the bridle is tight and the first anchor holding well we drop the second anchor to the bottom. We then move back to the ball and top chain, attach a heavy line to the ball and begin pulling hard with the dragger's engine to set the second anchor. You know everything is set well when the fishing boat's engine is at full power and the boat isn't moving. I always follow up this procedure with a dive on the mooring to see how everything settled and to recover the line we used to lower the second anchor.
Advantage of such a system is that you can recover the entire rig anytime you wish. Same is true of a big mushroom anchor, but I don't think a mushroom would stop a fishing boat at full power. Obviously, a concrete or stone block on the bottom is also more difficult to recover or move.
If you choose to go with such a rig, make sure that the top chain is of a length that it can not foul the down wind anchor. In my rig, the bottom chain is 85 ft with the top chain attached in the middle 42 feet from each anchor. Top chain is 44 ft so with the catenary it can never touch either anchor.
I've used the rig for years with no problems -- well, I did have one problem. As we were leaving for a multi-year trip away from the homeport, I decided to pull the mooring and store it ashore while we were gone. Problem was I had difficulty finding the anchors. They were buried 18" below the surface of the sand.
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