all comments posted thus far are incorrect. you can run any nvidia card with any other nvidia card, just not in sli, and do not connect the sli bridge to the cards. in the nvidia controle panel, under "comfigure surround, physx" it will display both cards that your running. in here you will be able to dedicate the 680 as the primary where it should show your 120hz monitor hooked up to it. and then the 660 showing the 2 x 24 in screens. you will be able to keep this defualt config for what you are wanting to do. under "change resolution", you will see all three of your screens, and can click each icon, and then set the resolution accordingly. i strongly recommend using a 1080p resolution for all 3 screens.
now, in regaurds to me knowing this, its because i currently am running a gtx 670 and a gtx 480 together on the same motherboard, but with no sli bridge connected. (doing this would fry my cards possibly). i have my 480 dedicated as physx, and have just my 40in hd tv hooked up to the 670. but i have also used a configuration the same as what the OP is describing, and if he checks what i described above he will be good to go.
as far as the 680 being able to run the 120hz screen at 120 fps consistantly? well, deffinatly not with 3 screens connected to it, but i know thats not what your wanting to do. hooking up your 2x24in screens (1080p) to the 660,should pose no performance hit, and your good here, even if they are 60hz, since when you game, they wont be displaying any game graphics. they will either be blank, or display your desktop, which is what the 660 is meant to do for you. but however, assuming that the 120hz screen is a 1080p resolution, in most games it should maintane 120fps, especialy if you can use evga card and precision x, cuase it has a feature in it that lets you set a sort of fps goal, and digitally overclocks the card as needed in order to acheive said goal. works pretty well that ive seen so far. but it doesnt alwasy work, so their will be games that will keep it under 120 fps, but turning down a few settings just a tad, will resolve this issue.
and lastly, the reason i say it would be adeal if you ran 1080p on all 3 screens is for 1, to simply make all 3 screens look consistant, and 2, running a resolution higher than 1080p on the 680 and wanting 120fps would not happen.
also, if you hook all 3 screens up to the 680, i do not think it would degrade performance, cuz the 680 should display them as blank or your desktop, and keep the 120hz screen on as your gaming screen. you may have to adjust a setting in the nvidia conrole panel inder "configure surround, physx" settings, and be sure nvidia surround is turned off. you should try it like this with out the 660 in the computer, and see if there is a difference compared to the 2 x 24 in hooked up to the 660. if not, which im thinking there wont be a difference since i once had this 40in and a 25 in hooked up to the 480 alone at one point, and all the games i played , layed the same if i had the 25 in plugged in or not, then id say you could probably sell the 660.
hope all this info helped some people out there.
and if any one thinks i may be incorrect about anything please comment.