Create a boolean valued parameter, call it say Should Expand. You can choose readable aliases, such as "Expand" for True and "Collapse" for False.
Show your parameter control and customize the look as desired
Create a calculated, call it, say Collapsable Foo, where Foo should really be the name of your true second dimension. Define it as
if [Should Expand] then [Foo] end
Make sure Collasable Foo is a dimension and use it as desired, presumably in place of Foo. You can edit the aliases for Collapsable Foo to set Null to a single blank to improve the way text appears when collapsed.
ATTRis an accumulator, so formulae can't return variouslyATTR([field])and[field]. - Him