There can be only one REFDES attribute. And you cannot copy it. You can configure on which layer you want it to appear, but you cannot make multiple copies of it.
If you want to have two REFDES showing up for the same component, one in the silkscreen layer and another one in the mechanical layer, then you have to go to the component properties (In Footprint edit mode) and add a new attribute, and call it REFDESML for example, and place a question mark (?) as the value. When you place the component, it will ask you for the REFDES, and then it will ask you for the REFDESML... you type the refdes text as the value for REFDESML.
Please note that this is not an automated process, when you transfer a design from Multisim to Ultiboard, Refdes are passed automatically. This new REFDESML will not be automatic, it will have to be added manually.
Nestor
