When I first unlocked the Legendary Dark Knight costume in the first Devil May Cry and noticed how Sparda had Luce & Ombra I thought that Ebony & Ivory were the same guns re-designed by Dante. Or that these guns didn't even exist and were added to the Sparda costume to suit gameplay. After all the similarity is quite obvious and Luce & Ombra do look like an earlier type of gun in comparison to Ebony & Ivory.
Luce & Ombra
Ebony & Ivory
If that was true, we could just assume, that these two ladies in the guns (which are the same ones in both L & O and E & I) are people only Sparda knew, and Dante just kept them in his version of the guns as well, because he either knows them as well (from Sparda or Eva) or just because he thought they looked cool.
However there was always the matter of Trish using Luce & Ombra, alongside Dante. In the first picture of Trish ever, she holds the guns. And continues to use them in games like Viewtiful Joe and Ultimate Marvel vs Capcom 3. She even uses them when you play with her as an secret character in Devil May Cry 2. I always thought she had them because it was necessary for the gameplay and they were bored to use new guns since they had already made them, but then in the anime and Devil May Cry 4, Trish used Luce & Ombra alongside Dante (who of course had Ebony & Ivory) making their existance official, and destroying any theory that suggested that Ebony & Ivory were actually Luce & Ombra redesigned by Dante.
Trish in DMC1
Trish in Viewtiful Joe
Trish in UMvsC3
Trish in DMC2
Trish in the Anime
And finally
Trish at DMC4
Therefore the only plausible theory I can think of is that Dante designed his guns after Sparda's. I have no idea who the women are. Both weapons use ergonomic wood grips which are inlaid with portraits of Victorian women; as befits their names, Ebony's is dark-haired, and Ivory's is fair-haired. I'm guessing there's no real story behind this, just a nice add to the guns' badass-ness or some kind of an easter egg the creators added. Like the " For Tony Redgrave. By .45 Art Warks" engravement which is a reference to the first Devil May Cry Novel, even though it isn't considered canon anymore. It doesn't mean anything, it's just there. Though interesting-ly enough Dante admitted in having used the alias Tony in the past (in the anime) making things even more confusing, so who knows.
It would be awesome to get a story a canon story about Tony Redgrave, Luce and Ombra and everything concerning the guns actually. Though with how things are going now *cough*reboot*cough* I'm not too optimistic about it. Only time will tell.
Good idea though, the thing that each of the gun represents someone Sparda might have known.