Bill,
I wonder about whether the lamination in the transom is sufficiently strong to take the load of the backstay without reinforcement. Perhaps someone who has done this will comment. It would seem to be a simple matter to glass in a set of reinforcement panels on the inside of the transom to accommodate a split backstay, but this would increase both the cost and complexity of the modification if such reinforcement were required.
I opted for two more bolts through the knee and a new thicker single backstay chainplate. I figure that if the original lasted 37 years this one ought to last awhile. I do have a turnbuckle type backstay adjustment device, that in it's current location requires backstay adjustment wrench that I made out of PVC pipe and a few miscellaneous pieces of hardware, but it all works just fine.
Also I like the look of the nice clean transom sans external chainplates. See photo