I've attached screenshots that go along with the directions, naming convention of images follows steps.
1) Your two subreport RDLs need to be in the same project.
2) Right click in the report body and insert a Subreport.
3) After you have inserted the subreport, right click the subreport and click on Subreport properties.
4) From the drop down, select the first subreport you want.
5) Do the same thing for the next subreport and this is what your report body should look like.
6 & 7) Create a parameter and set the available values to have a label of whatever you want the user to select and a value of your SubReport names in the project.
8) Right click the subreport1, select “Subreport Properties…” go to Visibility and click on “Show or hide based on an expression” and create an expression similar to below with your parameter name and subreport names. =IIF(Parameters!SubReportVisible.Value = "SubReport1",TRUE,FALSE)
9) Do the same thing for subreport2 changing the name in the visibility expression.
10) When the user runs the query, they are asked to choose which subreport is visible and that is the only one that shows in your report.
Everything works perfectly using the above method! Keep in mind that the queries for the subreports are run regardless if they are visible or not. If the report is taking a long time to render it may be due to one of the subreports that is running but not visible.