I wasn't able to get that running through the Rules Manager.
Here's the setTextBox function that I used to set the text content of one text box to another: setControlValue(findControl("CrmTextField_C004"), findControl("CrmTextField_C005").get_value());
You can also do it like this: findControl("CrmTextField_C004").set_value(findControl("CrmTextField_C005").get_value());
Additionally, you can find references to the client side API here
Please let us know if you have any more comments or concerns.