I think the Orc’s Bane is fine, even if the double attack is only against one Orc. It’s good for the Rogue, as mentioned, or by a character who regularly uses a Shield. It’s basically a Spirit Blade against orcs. It’s 2 and 2 instead of 4 at once. I believe statistically the two attacks are better than one attack of 3 or 4 dice, but I am not a mathematician, so I could be wrong. Also orcs are probably the most frequently encountered monster. It is also an artifact meaning it’s good against ethereal creatures as well. The introduction of ethereal creatures is my favorite addition from RotDM. It gives your seldom used artifact weapons more value to have. Even the Ice Queen’s Spear has some value now because of this.
I do like the idea of the Orc’s Bane working against the Orc Bard. It’s too comical to pass up on the off chance it actually happens. I do like the idea of changing things up a bit and having the hero’s race play some part. Such as if there was an Elf Wizard, they could choose the Elf spells as one of their possible options. Or allowing the default Elf Rogue the ability to use the Elf only items such as the Elven Bracers, the Elven Cloak of Passage, or the elf potions. You could even consider a Barbarian Scout (basically a scout from a barbarian tribe who has the exact same ability and stats as the standard dwarf). I would not mind if a Zargon player was using homebrew stuff like this. Though if you had other orc heroes you would probably want some advantage for them being an orc at some point like some orc-only items or a bonus for particular in-quest tasks like deciphering orc writing or something.