Does the inline-skate weight refer to one or both skates?

Your best bet is to contact the manufacturer or retailer for exact specs, especially since weight varies by size, as well as having limited comparative value when skates have very different wheel/bearing selections which make a big difference on total weight.

Aggressive inline skates weigh around 1kg-1.5kg, or around 2.5-3.5 pounds, per skate. See for example this discussion of weight, which while a bit out of date is still within reason for current models. As such, you can surmise that the site you linked is mentioning the weight for one skate.