I can honestly say I don't see a strong resemblance that would be enough to say they sound exactly the same with any album comparison. I do think the bands have a slightly similar sounds in regards to their music (maybe why I enjoy both bands?)
However, Nergal's vocalwork on Demigod is most likely layered in attempt at a more "brutal" sound (as someone mentioned earlier) and it comes off as being a bit sloppy (certainly not in a bad way). Karl's vocals have always been a bit more "brutal" than the average vocalist's but his vocals are more "fine" and "to-the-point" in that he doesn't really slur his words together as much as Nergal (on Demigod). Its rather hard to explain but I can definitely understand someone drawing comparisons betweens both vocalists but I would never say they sound identical.
Karl's voice is sometimes reeal deep, and sometimes on more like harsh shouting. His voice is pretty consistent throughout all of Nile's albums in my opinion (and yes, I do own them all). It is pretty commonplace in a death metal band to have deep vocals, and Nergal layers his vocals with a deep growl as well as harsh shouting on Demigod, to get an effect that is truly unique and really sounds fuck all like Karl. The vocals on Demigod and Black Seeds really just don't sound the same. I'm trying to get my head round why it is you hear the similarity? Anyway... I don't think this debate will be resolved so let's just agree both Behemoth and Nile are FUCKING AMAZING BANDS!!!