| Abstract
| - I discuss the constraints that could be placed on $\Omega_{\rm M}$ and $\Omega_\Lambda$ if it were possible to measure the redshift of an object at cosmological distance as observed by a second distant object. This hypothetical cosmological test has several attractive features. By a suitable choice of objects (in terms of distance from us and angular separation on the sky) a single measurement can constrain almost any given linear combination of $\Omega_{\rm M}$ and $\Omega_\Lambda$. These constraints do not depend on, or require marginalisation over, any other cosmological parameters (such as the Hubble constant) or any early universe physics. In principle, the test makes no assumptions about the objects involved and hence it is entirely independent of their physics and any possible evolution.
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