My hair is currently dyed a yellowy blonde. Can I mix a dark ash blonde with a medium blonde to tone down the yellow?

If you want your hair to have odd green tones, then yes :) But I don't think that's what you're looking for. If you have a blonde that is about the right level but just want to eliminate yellow tones, you need to use something with a violet base, not ash. Violet will cancel out yellow. A blue-based ash will create green (blue + yellow = green).