This is clearly a case for Batman’s Super Molecular Dust Separator, as featured in the 1966 Adam West film.
The only drawback is that the sugar and Splenda will come out speaking the wrong languages.
I was thinking chromatography, but seeing as how it’s a question of four different sugars (close enough) you’d have to do it reverse-phase. See if you can do it with something that won’t hurt you, like a water-ethanol mobile phase gradient. Then distill off the ethanol and lyophilize the remaining water. Skald didn’t say what scale, so I’d assume something like prep HPLC is out and you’ll be needing something big. Plus, you’ll need some other sort of detector as you’ll have no UV detection capability, so you’ll probably need a split into an ELSD or something similar.
I use that constantly, buying a 4 oz bottle for $17 that lasts for a year of sweetening tea.
I was going to say vibrate them, but you guys beat me to it.
Seems to me that Splenda’s mostly fluffy, and sugar’s relatively dense, so vibrating them should sink the sugar, and the splenda would be on top.
You could probably rejigger a Dyson-style cyclonic vacuum cleaner to separate them, but I suspect it would be a delicate adjustment.
“Electrostatic classifier.” High voltage powder-beam deflection. If they’re in contact, then the sugar grains are already charged to one polarity, and the Splenda is equal and opposite.
Pour them slowly in a thin stream in front of an operating CRT screen (spaced far enough that the powder doesn’t veer to stick to the screen itself.) The strong e-field should deflect them differently. Two separate piles should appear below the screen.
This might not give 100% separation, since grains might be clinging together. Repeat the pour several times?
Or: sedimentation rate classifier. Mix the powder into acetone, light oil, etc., then pour it into a tall column container such as a plastic pipe pre-filled with fluid. The components of the cloud should settle at two different rates and form two different clouds. When sufficiently separate, drain one off via a valve at the bottom. Bake off the voletile liquid.
Air-jet classifier: pour the powder-stream in front of a fan. Two piles form (or perhaps one pile forms, while the finer powder remains suspended and travels far from the first pile.) Or perhaps take the powder outdoors in a wide basket in a mild breeze and fling it upwards. The sugar grains fall again and remain behind, and a vast splenda cloud blows downwind. Benefit: colliding the powder would tend to knock any clinging grains apart.
Pretty much everything I ever did in pharmaceuticals was reverse-phase chromatography. It’s kind of a given in my mind!
The sucralose monograph linked to earlier in the thread has an assay method with UV detection at 190nm but allows for refractive-index detection as well. If sucrose is detectable at 190nm, then that would serve to separate the two based on elution time. I presume dextrose and maltodextrin could also be detected in that range, but a diode array detector with multiple UV wavelengths could would as well. You don’t really need to quantify, just qualitatively separate the components.
You’re right that scale could be an issue. I’ve only done pharmaceutical QC and product/method development, I’ve never worked with anything bigger. I haven’t even worked in the industry in 4+ years; I’m not sure I could take your ideas any further!
ETA: the monograph uses ACN in the mobile phase and diluent, but I suspect another polar organic solvent could work. The method could be tweaked with ethanol, I think.
Which part? The fact that it will still cause them, or the part about it not being as bad as regular sucrose?
If the former, it’s because the amount of dextrose and maltodextrin in an amount of powdered Splenda contains about 1/5 of the calories of an equivalent amount of sugar (as in, a teaspoon of sugar contains about 15-16 calories and a teaspoon of Splenda contains 3-4), and dextrose and maltodextrin are digestible by streptococcus mutans and other tooth-decay-promoting oral bacteria.
If the latter, it’s because, as Chronos said, the same amount of Splenda by volume contains only 1/5 the calories of the same amount of sugar, because the way they make that stuff it looks really “fluffy,” so to speak, and doesn’t pack nearly as well as as grains of sugar do, so you end up with a lot less weight in the same teaspoon or cup or whatever. And when bacteria like s. mutans have fewer calories of fermenting sugars to work with, you don’t get as much acid production.
This is the kind of awesome post that makes the Dope the best board around. You get some fascinating suggestions plus gems like “A vast splenda cloud blows downwind.” 
You never said the solution couldn’t involve killing you…
Ok, try adding “have someone” after the word “could” and then you don’t have to die in the process of separating it.
Dissolve it in water making a very concentrated solution, insert string, grow sugar crystals rock candy style. When that is done evaporate/boil off the water. Repeat if needed.
Have no idea if that will work however.
Baking Splenda as pointed out and as I remember is much lighter then sugar, this could allow you to pour it over a air stream of a fan and the Splenda should end up further away.
Don’t you just love these simple solutions in life, like the iPhone’s just don’t hold it like that. 
I’d go with the sieve idea. Splenda is finer than sugar.
Anyway, Splenda is a toxin (http://www.splendaexposed.com/). Instead, switch to Stevia - pure stevia not the kind mixed with anything else.
I buy mine from Swanson’s online. It is 100% Stevia.
There are many problems with the book you link to, but my feet hurt and so I don’t feel I can be appropriately insulting of it.
Yup, just like I expected. “It’s toxic because it contains chlorine”. Doesn’t anyone have any clue whatsoever about chemistry any more?
You teach college, don’t you?
Think back a little further. Think back to high school. Or, for that matter, think about the people taking Football Physics or Chemistry for Poets.
But “poisonous elements can combine into non-poisonous compounds” isn’t high-school level chemistry. That’s something that I’d expect my nieces, with single-digit ages, to know.
Mon ami, I think you’ve spent too much time in academe. It’s left you with an erroneous impression of the level of science knowledge of the average American.
One of my sisters has a master’s in social work, and she turns to me – ME – to explain extraordinarily basic stuff, such as why water can be made of hydrogen & oxygen yet not burn.
I’ve used Cecil’s explanation (or was it Joel Achenbach’s explanation? I can’t remember) that water is already burned – it’s like the ashes of a hydrogen fire.