Read a lot, and be cautious
Twin backstays are not necessarily better or more reliable than one. When you tune, you will need to put less tension on each, to keep the forestay balanced. If you use the same size rope, as many do who are trying to "beef up" their rig, the two backstays will be slacker than intended, and slacker stays are more susceptible to shock loads. If you go the other way, putting the same tension on each of two that you formerly had on one, then you will have to double the tension on the forestay. It also increases significantly the compression load on the spar, and the bending forces on the hull. If you move your shrouds outboard, you will need to lengthen your spreaders, adding even more weight high, or change their angle relative to the mast. Either way, your jib sheet angle will increase. Designing a rig is all about geometry, balance, materials, and experience. You might want to ask about your ideas on Brion Toss's bulletin board: www.briontoss.com. Good luck.
