update: They're now living happily in my local lake. I set them on the bank, and they scrambled in, and swam off before I could even turn my camera on. I actually feel really, really good about this. I waited around until they both came up for air, and when I flinched, they both darted back under like they hadn't been in captivity for the last four years! I hope they live out a long happy life, and I know this was the best for everyone involved.
:oops: