When a boat owner reaches the point that he starts to experiment with his rig, he starts into areas of sailing that have little sources to refer for guidance. Most of my experience with a double headsail has come from actual testing as well as studying old sailboat pictures of the 17th and 18th century. Today the universal production rig is the marconi or bemuda rig, but there was a time when these rigs were not used by all sailors.
The early Netherlanders were the most advanced sailing people, they ruled the seas until the English displaced them in a couple of epic sea battles. Until that time, the Dutch sailboats were known for having multiple headsails, and I have seen pictures of sailboats from that era that had bowsprites that exceeded the length of the hull. The effort to fill in the foretriangle resulted in very strangely shaped sails, sails flown without stays, etc.
How does this relate to our simple rigs? If you want to fly more sail area, it is quite possible to increase forward sail area, most easily by installing some kind of sprite. Remember that you are stepping into sail rig designs that required skilled, observant sailors capable of going forward and adjusting the rig to suit conditions. The idea that a crew would stay in the cockpit was not an option for these sailors.
If you have a fractional rig, the upper portion of the mast above the hounds can be thought of as extra materials to work with, while the portion of the mast below the hounds is the basic, backup rig. If one leads a new forestay from the masthead to the stem fitting, you will indeed be able to fly a sail on the new forestay. This will be best accomplished by installing an adjustable backstay, as the conditions you will flying a new and larger sail will probably require loosening the backstay to make the foresail fuller. You will not have a much larger sail going to the masthead this way. Note that the aft upper stays oppose the tension on the original forestay, the events involved above the hounds have little effect on the original forestay on a correctly tensioned rig.
A word about wire rigging. The fail limits of wire cable far exceeds our usual tensions, and most failures have easily seen evidence that can be found if inspected. I base this on experience as an aircraft mechanic, as a hang glider instructor, and a sailboat inspector. The worst damage I have seen to wire cable involved instantaneous shock loads of crashing student hang gliders. Even then, elongation and broken wires were easily seen. One could make a case that running a second forestay makes the basic rig stronger.
Forestay tension: consider that the foresails on the Cutty Sark were free flown, that is the sail were attached to lines that were run up, there was no inner stay and no hanks. This idea allows multiple sails to be flown across any two points capable of sustaining the load.
So I say go for it, I have. My 4 foot sprite allows huge sails to be flown on my 26D. I do pull on a lot of tension on my adjustable backstay. The interaction between the 3 sails is even more complex that a single headsail. I have extremely long sheets to allow the forward sail clew to go all the way around the boat as a lazy sheet. I lead the foremost sail sheets all the way back to the aft deck cleats. The boats makes way in the slightest breeze, almost making wind to the bewilderment of other sailboats that cannot move. Yes I have to remove the extra sail area as the wind increases, but what a great difference in sailing ability. I have recut the humongous genoa from a 36 foot donor sailboat to have a Yankee foot, which makes the original working jib more effective, while increasing effort in the foretriangle.
