Dominic,

I think the approach you take depends on what functionality requirements you have. If you have multiple controls that need to access data from each other, and those controls need to physically be in different locations (content panes) on a page, then IMC or Request.QueryString is what you should look into using.

However, if the controls can live inside the same DNN Module, then you shouldn't have to do anything special to allow them to access each other's data. In this case you can use something like a MultiView as Michael suggested, to add some organization to the logic behind showing and hiding controls in the module.