Craigslist might get you some responses, but if you go that route, do try to make sure the person taking them knows how to take care of them...
If you're in California, look up the CTTC: California Turtle and Tortoise Club. The chapter closest to you may be willing to take one or both. Beware, though, that they already take in literally thousands of RES a year, so they may be too full up to take on your pets. In which case, look up other reptile rescues in the area (google is your friend) and post an ad on Craigslist.
In the mean time, for goodness' sake, separate them!!! If you're so worried about them killing each other, why haven't you separated them?? You can find cheap aquariums on Craigslist or even just pick up a big plastic storage tub from Walmart or the like. Grab a spare heat lamp, something for it to get out of the water and bask on (a big planter pot set upside down with a tile/rock siliconed to the top works like a charm), and you're set until someone can come take it for you. It's not ideal by any means, but it sure beats having one of them DIE because you couldn't/wouldn't separate them!!!
Check out this great care website for Red Eared Sliders:
http://redearslider.com/. You should definitely read through it, even if you intend to get rid of both of them. This page in particular:
http://redearslider.com/startup.html has a chart that compares very cheap, "Basic" supplies to what you might normally get. A good list to go by if you're willing to spend the $$ to separate your two until someone comes forward to clean up your mess...