If Ryuuko's red blade can turn into Decap. mode, shouldn't Nui's be able to do so? I mean, who wants lopsided, weird scissors? And I'm sure Isshin didn't plan for the scissor to be seperated. Imagine Decapitation Mode Rending Scissors.
The scissors likely include life fibres in their design. It's possible that the transformations the scissors can do are similar to the transformations kamui are capable of, which appear to be limited only by the imagination of their owner and the amount of cloth available.
it is said they are made of complete life fibre during the anime (to lazy to search episode)
the decapitation made is indeed caused by an extension of transformation, and in turn the blade possesed by Nui did transform, both when combined into scissors to become double decapitaion mode. and while Nui used them to become a double bladed sythe like weapon.