In your scenario, you need two dropdown-list to display the data from two datasets, right?

In current version of SSRS, dropdown-list is on available on report parameter pane. You cannot use it in Report body. To display the data from dataset, you can use table, martrix, list, chart and guage. Please refer to the link below to see the details.

https://technet.microsoft.com/en-us/library/aa337396(v=sql.105).aspx

Anyway, if you want to display it on dropdown list, you can add two parameters in your report. Please refer to the link below.

https://technet.microsoft.com/en-us/library/aa337396(v=sql.105).aspx