Why don't you see any yachts with twin headstays? They seem like a great idea.
When sailing downwind, you can easily set two headsails poled out on each side giving you a giant balloon of a headsail with optimum control. Each sail will ride on its own stay, so there is no issue with lowering one of the sails should you decide to change course from a run.
You can also carry two headsails rigged on the stays for varying conditions. Imagine the wonders of having a 150% genoa on the port headstay and an 80% jib on the starboard headstay. If the winds are light, you can hoist the genoa. Should the weather change during your voyage for the worse, all you need to do is lower the genoa and hoist the jib. There are no sailbags to lug around or hanks to mess with, as the sails live on deck attached to their own stay.
From a security and redundancy perspective, twin headstays will be perfect! Imagine if you are sailing along with a single headstay and the headstay parts, the mast will become unsupported and risks falling aft and over. Some yachts have two backstays so that if one breaks, the other one will hold the mast up; why not have two headstays as well? If one headstay breaks, the other headstay will take up the slack and keep the mast up in the air where it belongs.
You might be wondering why twin headstays never really caught on, being how they make downwind sailing a breeze and offer the added security of a redundant headstay. The reason is because yachts do more than just sail downwind and twin headstays have some fundamental problems that led to their demise.
When it comes to sailing on a run, there is nothing better than headsails set wing on wing. They will create so much lee helm that the yacht will sail itself as you ride on a downwind sleigh ride. The moment you turn to windward, the twin headstays will work against you. Proper headstay tension is needed to keep the luff tight so that the yacht can point to windward efficiently.
Sailors quickly noted that the headsails would work on one tack but not the other, meaning that sometimes they could work to windward and othertimes, the headstay in use would be slack while the other headstay had all the tension. This problem is due to the geometric setup of the twin headstays.
The headstays were setup side by side, sitting next to the stem. When the mast is loaded by wind pressure on the sails, it will bend to leeward slightly. As it bends to leeward, the leeward headstay will go slack as the masthead moves to leeward and falls right behind the leeward headstay. At the same time, the masthead will fall away from the windward headstay which will increase the tension on the windward headstay as it takes all the force.
A twin headstay setup would work properly if you switched headsails as you tack, always using the headsail on the windward stay. This adds a lot of work, which will not be done because sailors went through the trouble of rigging twin headstays for the convenience, not for extra labor!
Since the leeward headstay goes slack, this must mean that the leeward headstay doesn't have enough tension. This led sailors to increase their headstay tensions to attempt to rectify this problem. Two really tight headstays made matters worse as the twin backstays were opposing this force and led to unnecessarily high compression forces on the mast.
The other problem with twin headstays is the hanks can get hooked on the other stay if the headstays are mounted too close to each other. They typically need 6 to 8 inches of clearance to avoid any interaction between the hanks on a sagging headstay and the other headstay. Aside from the hanks interfering with each other, you run the issues of the added windage from the extra stay. This problem is compounded should the owner try to rig twin furling headsails. The furled up massive headsail will cause so much wind resistance and turbulance that it will rob the working sail of its wind and efficiency.
If you ignore the downfalls of a twin headstay setup simply for the benefit of the structural redundancy, you will still be disheartened. Having two headstays for the sake of redundancy means that you assume that the headstay is going to break. If you feel this way, you should also have two backstays, and multiple shrouds, just in case the other stays break as well. Soon enough, your mast will look like a Christmas Tree with all the shrouds surrounding it.
Having one well maintained and properly sized headstay will provide you all the tension you need on any tack without all the problems associated with having side by side headstays. If you want to have the ability to setup two headsails with ease, you should consider a cutter rig, or setting up a solent instead.
Cutters and solent rigs have two headstays set in a line, where the stays still attach at the centerline of the stem and will not rob each other of tension, nor interfere with the performance of one another. The only problem with this setup is the small slot between the two stays will make tacking a large headsail troublesome.
With everything on a sailboat, there is always a give and take. If you plan to only sail downwind, a twin headstay setup would work well for you. If you plan on sailing at any other point of sail and still want multiple headsails permanently rigged, do consider a cutter or a solent setup.