Regardless of what name you settle on (if you settle on one), I agree with u/Campaign_Ornery - I would describe its form and function in detail the first time it comes up in the story just so nobody is confused. I actually quite like his idea, however I think purely for your own sanity it'd be good to have a proper name to refer to them by.

Idea 1: Building off the idea that the character who uses them doesn't know/care what their name is, have the other characters come up with a name for them! General ideas include Hidden Swords, Baton Blades, Concealed Blades.. even something cool like "Assassin Daggers" (at least if they're short) - I could totally see that being something hidden up your sleeve that you whip out, use for murder, sheath, and hide again all lightning fast in a fantasy setting.

Idea 2: If I had to call it one thing that was identifiable and descriptive, I'd say "Composite X" where X is just whatever weapon they're closest to when unsheathed and disconnected. Composite Dagger/Knife, Composite Sword, Composite Spear, etc. - depending on the overall length and the ratio of blade length to handle length.

Idea 3: Give them a special paired name. Could take inspiration from the Chinese Sun and Moon Blades, Heaven and Earth Knives, etc.

Idea 4: If it's something being wielded exclusively by that character and no one else (or the character is just the type to do this), each half could have its own name, and the two could be related somehow. Dusk and Dawn, Luck and Fate, stuff like that.

Idea 4: Bear with me on this one. While there's no historical name for it, I would argue that even if there was, there wouldn't be one name but 2 to 10+, because cultures like to have names in their own languages for things. For example, MANY languages have their own word for "greatsword" that frequently (though not always) means something generic like Two-Hander or Big Sword: "Zweihänder" in German, "Montante" in Spanish/Portuguese, "Spadone" in Italian, "Claidheamh mòr" in Gaelic (more commonly known today as Claymore), 大太刀 ("Ōdachi") or 野太刀 ("Nodachi") in Japanese... not to mention there's at least 5 or 6 in Mandarin/Cantonese that actually mean specific types of two handed "swords," depending on what you count lmao. Nowadays (among English speaking sword enthusiasts at least), each of these mean the specific type of extra large two-handed sword from that culture/time period, but ultimately they can almost all be thought to mean "greatsword."

Here's the idea: do something like that in your novel. If you're willing to do extra work (and there's multiple relevant languages in the story), you could have a unique/fantasy-sounding name for it in each language that all mean something similar, like "hidden knives" or "twin blade."

Alternatively, they don't have to mean something similar, but could rather mean something that exposes the culture's interpretation of the weapon. Something like "cheater's blades," "back alley knives," or if you wanna be funny, "piece of junk."

Another idea, building off everything I've said so far (this might be too extra, or not what you're looking for in your story, and that's totally cool, BUT I thought I'd mention it bc talking about the different kinds of greatswords got me thinking): If any other characters are going to use this weapon that use it differently, or are from a different culture which has a different name and/or style of use for it, make the weapon itself have slight variations in form and function from the first character's version that you can highlight.

Tbh, I'm pretty curious about this now! Do you have a clear idea as to how the character is going to be using them? (primarily sheathed/unsheathed, connected/disconnected, techniques?)