I’m guessing Ushimaru was probably a two sword style user. Ushimaru is based off a real person from the Minamoto clan. The Minamoto clan was famous for their two swords Higekiri and Hizamaru, called the brother swords. These are likely the names of Ushimarus two swords. When these two swords were separated between brothers the clan fell into ruin, however, when they were reunited under one brother the clan prospered. The two swords were also renamed many times throughout their history. Interesting to note is that Higekiri was once renamed Onikiri or demon cutter.
Which is a pretty big deal when u think about it considering food literally works as senzu bean in the story. (luffy the MC being the prime example getting healed/restore to full power by eating meat) Who knows how many days passed without him having any food (even before yamato was put in there with them).
Here's something I wanted to share from the Zoro FC I feel is relevant here as well. It pertains to the idea that Onimaru didn't recognize Zoro to resemble his old master. I justify why that isn't necessarily the case, and Oda merely doesn't have Onimaru point out Zoro's similarity to Ushimaru to the reader, but there are perhaps hints he is aware.
Something I like to bring up is the idea that Onimaru did not connect Zoro's resemblance to Ushimaru. If we go back to the chapters he first confronted Zoro, his behaviour around what was a total stranger, was off.
All he does is take back Shusui and didn't attempt to steal Zoro's other swords. He later lectures him about Shusui's importance, and later expresses disappointment in Zoro's response to the lecture.
Now think about it, Onimaru had just seen Shusui after over 2 decades missing, and he stumbles upon it in the hands of this stranger. As far as he should be concerned, Zoro was a no good thief galivanting with the country's treasure. Despite that, he didn't attempt to steal Zoro's other swords, just Shusui. He states he "over-estimated" Zoro when he had never met him till that point, suggesting he had expectations for Zoro once he laid eyes on him. He was then seen smiling after Zoro defeated Kamazou, only stealing the latter's weapons. (Though defending innocents probably left a bigger impression on him at that point, but still worth noting.)
Onimaru was by Ushimaru's side for years. If anyone could spot a resemblance between Zoro and his late lord, it'd be him. And based on how Onimaru treated Zoro vs how he treats other ruffians he barely knows, has me leaning on the idea he could see the similarities, and that's what informed him on Zoro's character initially. Even moreso, he's a fox and likely recognized Zoro to be related to Ushimaru based on his scent.
Something I also want to add that I didn't in the original post is that Kawamatsu notes Zoro and Onimaru weren't trying to kill each other. We know why Zoro wasn't, he just wanted Shusui back, but we don't know why Onimaru was holding back. Unless he feels Zoro is Ushimaru's son, then it would explain it.
Needless to say, I wouldn't dismiss this as a plothole or inconsistency if Zoro turns out to be related to Ushimaru. We simply don't have the full picture.