You'd have extremely low entropy though. If I write an auto-signup script that just selects one or the other randomly, I'm still getting 50% of my botted accounts through.

Sure you can add more, but how many can you add before users get upset about doing the same thing over and over again? Three? Four? At four times I'm still looking at a 6% chance of getting an account created if I just select randomly. That's plenty.

Suggestions to use things that humans can easily distinguish but computers can't as captchas come up often. However, being only human readable isn't the only requirement for a captcha - another requirement is that it can't be computer guessable. If a computer can guess correctly with a high (>1%) chance of success, then the captcha is pointless.