I've started re-learning Spanish in college (took it in the 8th grade) and I'm picking it up fairly quickly again (through plenty of studying on my own, of course!).
Granted, I can comprehend a fair bit of Italian based on its similarities to Spanish and what I've actually studied of it before.
So my question is, would it be a bad idea to start studying some Italian as well? I can imagine one language sneaking into one of my inner-dialogues (as have some other languages I've had previous exposure to), but I'm usually fairly careful with what I actually say and put down in writing and I don't imagine that, due to the similarity of the languages, it would be a fatal mistake if I did happen to substitute the wrong word.
And I'm not sure I could avoid that completely anyway even if I did learn Spanish very well beforehand.