This image from the famous painting of Lord George Murray shows the side-by-side configuration, with a metal chape and locket below the by-knives:



That's very similar to the scabbard of this Scottish "whinger":



Here's another close-up from a period painting (Lord Duffus) showing an all-leather sheath:



This original brass-hilted dirk's all-leather scabbard shows side-by-side slots of molded leather: