We report an efficient implementation of a spatial two-zone method for solv
ing Schrodinger's equation numerically, using a canonical basis set decompo
sition, and also its application to two-electron wave functions in a study
of double multiphoton ionization. We make the first calculation of the time
-dependent degree of electron correlation for this poorly understood proces
s. Our results show a particularly sensitive window of intensities coincidi
ng exactly with the well-known "knee" regime in ion-count data.