---->magazine articles ----> "double headed rig"
Probably the ONLY authoritative article ever written about the interaction, etc. of staysails flown UNDER a genoa.
Specificallly the article suggests that staysails should be especially geometrically (draft position, etc.) constructed/shaped to allow for the optimization of sail interaction between the genoa and main ... otherwise adding a staysail (or bowsprit) will probably not gain you very much ... and will result in god-awful sail shape of BOTH genoa and staysail.
The alternative with respect to a modern 'marconi' rig would probably be best served by adding a 'solent stay' - an inner secondary 'headstay' that is very close in proximity to your current headstay ... although using two headstays opens you up to the rigging complexities of having two headstay loads .... and being reactively stress balanced by only ONE backstay.
Simple Speak - is your (single) backstay able to take the required loads from TWO headstays? If each of your intended headstays (each) are not set to the (typical) 12-15% of tension - the luff sections of sails used will NOT match the sag of the stays when fully loaded ... and the backstay will be 'double loaded' if you apply 12-15% in EACH of the headstays .... meaning the cut luff section (luff hollow) of the sails will need recut radically different than if the boat has ONE headstay. It gets MORE complicated than this but hopefullly you will get the 'idea' of simply 'adding' a headstay and not considering what it will do as regards the luff geometry requirements of 'typical' sails at work with stay tensions that are quite different than the 'usual' working design of 12% (of ultimate tensile strength).
Simply adding a stay here and there and not fully 'balancing' the rigging reactionn loads will result in very 'loose' (1/2 tension of 'normal') headstays ... and that will cause some very BAD sail shapes for both the genoa and the added staysail. You will need to ADD: either intermediate stays (or running backstays) and find a means to 'double' the strength of the current backstay .... otherwise the mast will probably become VERY distorted when sailing at 18+knots of wind, the boat wont 'point' worth a damn because the headstays are way too loose, etc.