my initial thought was much along alex's: 'get someone else to do it.' there's got to be some magick school around, eh? heh heh.
this sounds like one of those stories where magick systems are just too complex for me. i don't like magick explained all that much. oh, well, that's just me (i like magick to go awry sometimes, too).
it crossed my mind as i skimmed this: russian roulette. after making a deal with whatever deity controls that brand of magick, in exchange for two people dying it just takes, say, five years off everyone's life... but one still has to bite the big one.
so, the orb in the middle the gawd/daemon leaves behind fills with power as the spell is working, and periodically zaps the people, testing their strength, resolve and ability, sucking years away from them in the process, until, at last, it determines who's the overall weakest. naturally the most hated in the group is the strongest, but is poisoned to weaken them, thus making them the target. or something like that. it could happen in, oh, about fifty billion variations.
cool idea? yeah, i like it. the practicality of it? fairly slim. the usefulness as it relates to your story? somewhere hover around nil. still, you wanted an idea, thar she blow. (actually, you weren't asking for ideas.)
is it cheating? well, considering you set the rules, yeah, it's cheating. if you don't want but one person to have to die, why go into this whole side thing where they figure out ways to cheat the devil his due? unless, of course, that's a major point of the story, otherwise it seems like filler to take up space and show how clever they can be, which isn't a bad thing, how clever they can be, but i'd guess that at this point in the story we'd already have figured that one out, eh?
i guess my question is why is it so necessary to have two die and figure out a way to beat the system when you, as the rule setter, can just make it one person? i mean, it's not like you can't change the rules any way you want, lol. you sound kinda like you've written yourself into a corner and somehow feel as if you have to spend twenty pages writing your way out when you don't have to at all. from what i gather you've got ghosts returning from other realms to impart information and i'm wondering if all that's necessary? i want their deaths to be horrible as a result... this is how good wizards are made... a daemon's hand materializing from nowhere to rip them to shreds before dragging their screaming soul to hell. last thing i want is for one or two of them to faint and just quit breathing. bah! what is this, a kiddy story?
every time they get zapped, they should pee themselves. one should really get off on the feeling, too (hey, wizards are freaky people, right?). these guys should suffer for their sin of killing with the dark arts. suffer, burn, get knocked down and have to get back up while their hair is smoking, all the while still having to perform the spell while daemons laugh in the background for the soul food they're about to feast on, probably taunting the others in the meantime. 'you're going to die, you're going to die, you're going to die!' and all that.
*that* sounds cool to me.

and, yeah, i'd be disappointed if no daemon grabbed one of the girl's rack. i mean, it's only fair that for taking someone's life these guys have to suffer all the humiliation and degradation they can in the process despite their 'pure' motives (and let's be honest, we're not exactly talking about 'harry potter' here). by the end, they drag themselves out of the chamber, tortured to the point where they don't even really care who got killed as long as it wasn't them. it's not as if the walls aren't sprayed with everyone's fair share of blood. such is the price of evil in the name of good, though, wouldn't you say?
or you could just have the main MC flat out lie to them and say it kills two when it only kills one just to test their devotion. that's a jerky thing to do, though. alternately, the spell kills however many people it kills who can't endure the process, be that none or all.
there's two ways to look at it: outright sacrifice, someone knowing it's them; and risk, everyone knowing it *could* be them. with sacrifice you effectively remove suspense, with risk you remove that long emotional goodbye (it's easy to see the previous scene where everyone is saying adios to the chosen 'victim.' you could have it both ways, where you have the person sacrificing themselves not be the person/people who die at all.)
you do understand that i could go all day long like this, don't you? lol.