Tapered root threaded hollow perforated interbody spinal fusion implants are disclosed for placement into a disc space in a human spine between adjacent vertebral bodies. The implants have opposite arcuate portions with lockable screws passing therethrough for engaging each of the adjacent vertebral bodies. The implants are adapted for use in side-by-side pairs such that a portion of the circumference of a first implant nests within the circumference of a second implant, so as to have a reduced combined width.