Presumably the penis, but valid question with an answer that could go either way. He is shown to also have a clitoris, and on the vulva, the urethral opening is between the clitoris and the vagina. So he would probably have two urethras.
Also fun fact: the penis and the clitoris are analogous structures, so in an alternate world where the Duke is a top, he would probably have two penises.
This question would be easier to answer if he didn't have a penis and a clitoris, and instead had the penis positioned as a macroclitoris
Oh my god you guys need sex ed. Nobody pisses out of a vagina. We all have a uretra, which is yet another tiny hole that's either on the penis or a bit up high the vagina. Since he has a penis, his uretra is very probably there. I'm so sorry you were never taught that. The uretra is also why you need to pee after sex (even if there's no penis in vagina action) if you have a vagina, cause during sex fluids get there and will cause a urinary infection if you don't flush it out by peeing. Those fluids are perfectly ok in the vagina but not on the uretra.
I would assume that, as having a vagina (no just the hole, the whole thing) also includes having an urethra, he could have two urethras unless it's specified that the vagina he has doesn't have it (which is probably the answer). However it's a manhwa that doesn't make sense in the first place why so serious
Which hole does he piss out??