Constructing two-writer atomic registers

A two-writer, n-reader atomic memory register is constructed from two one-writer, (n+1)-reader atomic memory registers. There are no restrictions on the size of the constructed register. The simulation requires only a single extra bit per real register and can survive the failure of any set of readers and writers. A complete proof of correctness is given. Several obvious ways are suggested to try to extend this algorithm to more than two writers, none of which work. As an example, it is shown how a natural extension of the two-writer protocol fails