What is known about the description of the Zul-Fiqar in classical Islamic sources? Not much or better nothing from first-hand sources. Yes, it is correct that the famous Shia version of ‘Ali’s (r) sword, the so called Zulfiqar can be found in many Sunni circles (especiallay Ottoman relics), however, none of these serve as evidence (also not movies), especially since according to the language of the Arabs no two-edged sword is called Zulfiqar.
It is not quite clear why the sword is assumed to have been bifurcated or what, exactly, that means. It is rather quite likely that the sword was not even remotely similar to what was made of that later. Bifurcated, two-pronged or double-bladed swords other than the zulfiqar, to the best of my knowledge, have never been found or were mentioned in ancient sources. The reason for this is simple: It does not make for a better sword.
The real zulfiqar was rather not two-bladed. In all probability it wasn’t two-pronged either. There is little to no doubt that Iranian myths contributed to the “reconstruction” of the sword, in memory of their hero and Shia saint (Abu Lu’lu’ah al-Majoosi) they have attributed his dagger (with which he stabbed ‘Omar and a dozen other Sahabah) to ‘Ali, all in order to give their Sassanian-Zorastrian grudge a thin guise of Islam. Some Shia scholars even admit that according to their own mythical beliefs, the “real” Zul-Fiqar is with their hidden saviour (“Mahdi”) and that it was the Shia who had influenced the Ottomans who in return championed the Majoosi Zulf-Fiqar version of the Shia. As a matter of fact, many Iranian ultra-nationalists who despite Islam have a soft spot for Shi’ism (for obvious reasons) and always mention that the Zul-Fiqar is actually the dagger of their lowlife hero Abu Lu’lu’ah. These ideas and grudge are deeply rooted in Shia Persian Majoosi culture:
The following video beautifully proves how Shia symbols have found their way into our culture, we must put emotions aside and purify our heritage from Rafidi myths, we own that to the Ahl Al-Bayt رضوان الله عليهم or in he words of the Iraqi Sheikh Taha al-Dulaimi:
“Expose the Shia, expose their links to the Persian Majoos and Yahood and purify Ahlus-Sunnah from their filth!”