Set default value of PDF form field as the value of another field in the same PDF form

Does anyone know how to make the default value in a field be equal to another field.

For example if I had a field called "Name" and I wanted that field contents to be the Default value in a field called "Name2".  The idea is not having to input the value twice but leaving the possibility to change the value in field "Name2" "the default value" to another value if desired.