  We report the discovery of two concentric Einstein rings around the
gravitational lens SDSSJ0946+1006, as part of the Sloan Lens ACS Survey. The
main lens is at redshift zl=0.222, while the inner ring (1) is at zs1=0.609 and
Einstein radius $Re_1=1.43\pm0.01"$. The wider image separation ($Re_2=2.07\pm
0.02"$) of the outer ring (2) implies that it is at higher redshift. Its
detection in the F814W filter implies zs2<6.9. The configuration can be well
described by a total density profile $\rho_{tot} ~ r^-g'$ with $g'=2.00\pm0.03$
and velocity dispersion $\sigma_{SIE}=287\pm5\kms$. [...] We consider whether
this configuration can be used to constrain cosmological parameters exploiting
angular distance ratios entering the lens equations. Constraints for
SDSSJ0946+1006, are uninteresting due to the sub-optimal lens and source
redshifts. We then consider the perturbing effect of the mass associated with
Ring 1 building a double lens plane compound lens model. This introduces minor
changes to the mass of the main lens and allows to estimate the mass of Ring 1
$(\sigma_{SIE,s1}=94\pm30\kms)$. We examine the prospects of doing cosmography
with a sample of 50 double lenses, expected from future space based surveys
such as DUNE or JDEM. Taking full account of the model uncertainties, such a
sample could be used to measure $\Omega_m$ and $w$ with 10% accuracy, for a
flat cosmology.
