I really liked your text. It was a pleasure to read. What I was thinking about: You speculate that the salinity on both worlds would be lower than in earths oceans. In the textbook Fundamentals of Geology (German: Grundlagen der Geologie, p. 108-110, Bahlburg H, Breitkreuz C, Springer, 2008 - 3. Edition) the authors state that the salt content of earths oceans remained rather constant during geological time spans. This is surprising as large freezing events withdrew vast ammounts of fresh water from the oceans and large evaporation events and consecutive salt precipitation withdrew large ammounts of salt from the oceans. As you also pointed out salts are also leached from the land surfaces into the oceans. So one would expect the salinity of our oceans to have changed over long periods. Bahlburg and Breitkreuz highlight the role of the midlle oceanic ridge to answer this. If the salt content of the oceans rises Albitisation (Albite) will increas and withdraw Na+ from the water whereas Cl- will recombine with other cations, keeping salinity stable. If the salt content goes down Albitisation will slow down and again keep salinity stable. At hte middle oceanic ridge there is a conatant supply on which Albitisation can occur resulting in the salinity found in our oceans. If this is true your two double-earths should have salinities governed by the parent rock material and not controlled by absence of land masses or the mixing speed of the oceans. Unfortunately Bahlburg and Breitkreuz give no citation for their claim. Only suggestions for further reading at the end of the chapter. edit: Clarified the role of the oceanic ridge. edit: Replaced Albitisation link, which needed registration with a free access (and actually even better explaining) link.