We introduce a new method for computing photo double ionization (PDI) cross sections for two electron atoms. It is formulated in terms of the hyperspherical radius R and relies upon a combination of R matrix techniques in the inner region R≤R0 with a semi classical approximation for the R motion in the outer region. We present a first application of this method to the PDI of He within a model of reduced dimensionality where r1=r2. It demonstrates the validity of our numerical scheme and provides a first quantitative estimate of the energy domain of validity of the Wannier mechanism.