Hello,
First I'd like to recommend that you read an old Fair Leads at this address:
http://www.briontoss.com/education/a.../miscapr99.htm. It's titled "Why Twin Backstays are Evil," which should give you an idea of my opinion. A forked backstay, on the other hand, can be preferable to a single one for a number of reasons: it distributes load over more of the hull; it can allow for more aft cabin space on hulls where the backstay comes to deck well forward; it allows for "frapping" adjusters on small craft, and 2:1 purchases on larger ones; gets the stay off the back of the helmer's head; and so forth. But if the transom is up to the load, simply offsetting to one side of the middle, as someone suggested, gives you the simpler single option. And a convenient handhold to boot. On boats like Concordia's, the single backstay is just a tiny bit off-angle, to miss the mizzen, and lands just far enough forward that it doesn't interfere with the mizzen boom. But most yawls and ketches will split their backstays.
A forked backstay is also the best thing for a double-ender with outboard rudder. There, and on most other boats, try for an angle between the legs of 40? or less. 30? is optimal, if it doesn't put the link too far up. Or even if it does.
Fair leads,
Brion Toss