The Collaborative International Dictionary of English v.0.48:

Coextensive \Co`ex*ten"sive\, a.
   Equally extensive; having equal extent; as, consciousness and
   knowledge are coextensive. --Sir W. Hamilton. --
   Co`ex*ten"sive*ly, adv. -- Co`ex*ten"sive*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

coextensive
    adj 1: being of equal extent or scope or duration [syn:
           coextensive, coterminous, conterminous]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "coextensive":
   aligned, analogous, coequal, coextending, coincident, coincidental,
   coinciding, collateral, concurrent, congruent, correspondent,
   corresponding, coterminous, equal, equidistant, equispaced,
   equivalent, even, lined up, nonconvergent, nondivergent, parallel,
   parallelepipedal, parallelinervate, paralleling, parallelodrome,
   parallelogrammatic, parallelogrammic, parallelotropic,
   synonymous