Try this out.
suppose your report query is like this.
--introducing one more column that define the number of rows in you selection and assign then a numeric value through 1 to 50
row_number() over(order by (select 1)) as 'r_num'
So, Above query have your result set required for report.
Create New Report->create your dataset->on report body create a table area.
1. In grouping pane Create Parent Group to your Default and write the grouping expression.
=iif(fields!r_num.value<=10,1,2) dividing your data on to two different partition.
2. Select Detailed Row from table area->go to row visibility->choose "Show and Hide based on expression" and write the below expression
and "Display can be toggle by" Select your Parent Group Name.
3. Select The Grouping Row from your Table in property window go for the property "InitialToggleState" and write expression.
if you feel this is as your ans then mark it.
if you require supportive report file to the above example do let me know.