In order to scheived the required functionality, you can attach to the OnRowSelected event handler, and iterate through the RadGridClientObject.MasterTableView.SelectedRows collection.
This will give you access to all the rows currently selected, and you can then access each cell individually, to get its value.
After that you could initiate an ajax request
to add the selected values in the second RadGrid DataSource (if the both DataSources has the same structure).
I hope this is a suitable approach for you.
the Telerik team