Create tab for related content types
Most simple way (i.e., without coding) would be to use Panels module, and create a node type view page override for the first content type.  Then use a nodereference field to make the first content type reference a node of the second type.  Lastly, use Panels' context system to generate a new context of that referenced node, and display it in whatever means you want on the overridden node view page.