Set the current value of a FileUpload control

I have a 2 FileUpload controls.
Once the user selects a file in the first control, I want to set the default value for the 2nd control to be the value from the first control.
For example, if it were textbox controls I would just use TextBox1.text = TextBox2.text.
How do I do this for FileUpload controls?  I tried a few different properties, such as FileUpload1.FileName = FileUpload2.FileName but I get an error saying that they are Read Only.
 
Thanks.
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