  Double source lensing provides a dimensionless ratio of distance ratios, a
"remote viewing" of cosmology through distances relative to the gravitational
lens, beyond the observer. We use this to test the cosmological framework,
particularly with respect to spatial curvature and the distance duality
relation. We derive a consistency equation for constant spatial curvature,
allowing not only the investigation of flat vs curved but of the
Friedmann-Lema\^itre-Robertson-Walker framework itself. For distance duality,
we demonstrate that the evolution of the lens mass profile slope must be
controlled to $\gtrsim5$ times tighter fractional precision than a claimed
distance duality violation. Using LENSPOP forecasts of double source lensing
systems in Euclid and LSST surveys we also explore constraints on dark energy
equation of state parameters and any evolution of the lens mass profile slope.
