Double USM doubles your sharpening control: you can set a Radius, an Amount and a Threshold for Dark Halos, and different ones for Light Halos. As Color-Correction expert Dan Margulis has demonstrated, Light Halos tend to be visually more offending and need to be toned down – both in their intensity (Amount) and in their thickness (Radius).
A heavy handed pre-press default USM is 500-1 (Amount-Radius), that you can always tone down with Layer’s opacity. Double USM gives a more natural effect with (400-1.5) for Dark Halos and (250-0.7) for Light Halos – about half intensity and half thickness.
Also known as High-Radius, Low Amount: used to give shape and sort of a 3D look. A starting point could be (50-50) for Dark Halos and (30-30) for Light Halos. As you see, the Radii are unusually high, while Amounts are below the values you would expect.
With Double USM nothing prevents you from experimenting: a creative sharpening might imply a Standard USM for Dark Halos (say, 400-1.5) and HiRaLoAm for Light Halos (say, 30-50) in order to get the best of both worlds!
Create and apply your own Sharpening Presets! Mind you, when it comes to Sharpening there are no perfect receipts nor absolute values: it always depends on the image’s resolution, subject, and eventually on your own taste!