This is a printer property. So either configure the default printer to print double sided, or make a new printer and use that to print to. Not sure if that's workable in your situation, but definitely the easiest solution if it is.
I don't think there's much control over the printer from LabVIEW... I'd guess that would require power shell or .NET commands.
It looks like it can be done via some .NET calls.
With a .NET constructor, I was able to navigate to System.Drawing.Printing.Duplex and then there's a property node for value.