  The magnetic design is a basic aspect of the superconducting magnets for
particle accelerators. When dealing with single aperture cos-theta type
dipoles, at the first order, the design can be performed with an analytic
approach based on a sector dipole approximation followed by a numerical
optimization. For double aperture dipoles the magnetic cross-talk between
apertures makes this approach unfeasible. We have developed a semi-analytic
model, which starting from a sector dipole approximation, allows to consider
the cross-talk between the two apertures. We also demonstrate that the iron
yoke contribution to harmonics, although dominant, does not change the optimal
configuration found in its absence. As examples, we show two possible
electromagnetic designs for the D2 dipole of the High Luminosity upgrade of
LHC. The semi-analytic model can be generalized to a larger class of magnets.
