Key points are not available for this paper at this time.
The initial-value problem is posed by giving a conformal three-metric on each of two nearby spacelike hypersurfaces, the proper-time separation of the hypersurfaces up to a multiplier to be determined, and the mean (extrinsic) curvature of one slice. The resulting equations have the same elliptic form as in the one-hypersurface formulation. The metrical roots of this form are revealed by a conformal ``thin sandwich'' viewpoint coupled with the transformation properties of the lapse function.
James W. York (Mon,) studied this question.