Ken,

There is description of a case:

1. On this page, i have 3 Uploader element.

2. First element has an attribute Visible="False".

3. Browse files for upload using second and than third element.

4. Press button Submit.

5. Uploading doesn't start.

This case don't work for any element (for example, 1st element has attribute Visible="True", 2nd elemtnt has attribute Visible="False"). 

But, if you  change attribute Visible to "True" for all elements, the upload running successfully.