...I love you for coming up with that idea!!!!!!!!!! Now it makes even more sense!!
Lol, I actually came up with that thought yesterday too, but dismissed it as being too obvious.
But overthinking it again now, it certainly does have some credibility to consider that Xemnas actually
knew what is hidden inside Sora.
He expected to win their last fight and doesn't really care about Sora.
The phrase "Can you spare a heart?" can be interpreted in different ways, and to us "viewers" of the story it can come across as very ironic since we know BBS, as Sora
indeed has a "spare" heart.
I said before Xemnas knew about Ven's heart in Sora.
So how could be possibly know?
Aqua? Nope, not possible since Aqua doesn't even know Ven's heart left its body after she left him in the chamber of waking.
But lets think about another person...
I don't remember it exactly, but I recall it was said in Days Xemnas created Xion as a fail-safe if both Sora and Roxas prove useless to his plan. Yet he also wanted Xion to absorb Roxas and become a "second" Sora.
Xemnas is also very knowledgeable about hearts, so he should know about that hearts influence appearance, and if he knows how Ven looks like he should instantly have recognized that Roxas, Sora's nobody, does look more like Ven than Sora and therefore must harbor his heart.
If Xemnas's plan had succeeded and Xion absorbed Roxas he would probably have used her until his Kingdom Hearts was fully complete, and then extract Ven's heart from within Xion, as he seemingly needs no keyblade for that (evidenced by his "Invitation to nothingness"-attack in KH 2), and then destroy her since with KH complete he would have no more use for her.
Later in KH II Xemnas obviously knows that Roxas has rejoined with Sora, bringing Ven's heart back into Sora with him, so him trying to suck out Ven's heart from within Sora would make sense.
However, what in this assumed scenario doesn't make sense is why Xemnas allowed Saix to try and destroy Sora after the artificial KH was completed, as Saix asks: "So I can end this charade?" and Xemnas replies "Indeed!"
If he really knew that Sora harbors Ven's heart and really wanted to secure it he wouldn't have allowed that, would he?
Probably "Can you spare a heart?" is really just meant as an ironic allusion, lol.