  Sets of orthogonal basis functions over two-dimensional circular areas--most
often representing pupils in optical applications--are known in the literature
for the full circle (Zernike or Jacobi polynomials) and the annulus. This work
proposes an orthogonal set if the area is two non-overlapping circular pupils
of same size. The major free parameter is the ratio of the pupil radii over the
distance between both circles. Increasingly higher order aberrations--as
defined for a virtual larger pupil in which both pupils are embedded--are fed
into a Gram-Schmidt orthogonalization to implement one unique set of basis
functions. The key element is to work out the overlap integrals between a full
set of primitive basis functions (products of powers of the distance from the
mid-point between both pupils by azimuthal functions of the Fourier type).
