I guess the knife was too dull to finish the second one.
Syrian Fight “Islamic State (IS, former ISIS)” beheading a Syrian soldier in the Raqqa governorate. The incident took place after IS took control over the “Brigade 93 army base” on August 8th, 2014. The IS took Brigade 93 after fierce fighting and a triple suicide bomb attack. In the fighting for Brigade 93, “36 soldiers were killed, some of whom were beheaded, while others died in the fighting or in the initial triple suicide bombing” that launched the attack.
They seem to have had more success here. ISIS signature seems to be hanging the heads on a fence in town for everyone to see: