If you load your table as a logical table in Qlik rather than a se of disconnected fields, there is nothing special needed. When you select Date Number = 2, the corresponding values of the Date Value become "possible" (indirectly selected,if you wish).
(Not clear from your description why you need GetFieldSelections(), if you have a reasonable data model.)
I see from your example that you want to create two variables containing Date Value, first and second, and there are always two(?). There are other ways, for example:
v1 = date(min([Date Value]))
v2 = date(min([Date Value],2))
v1 = date(FirstSortedValue([Date Value],[Date Value],1))
v2 = date(FirstSortedValue([Date Value],[Date Value],2))
You maybe need some conditions in case both dates are the same.