I don't believe those scissors would work particularly well @Spravotchka. I have a feeling they'll fold the carpet fibres/pile into the blades, and they'll become jammed. Traditionally, a snap-off knife would be the best tool to use. When it gets dull, snap a blade section off to ensure it stays razor sharp.
If you have the option, it's best to cut from the underside of the carpet instead of through the pile.