Scott Young -- Language switching
Benny the Irish Polyglot -- How not to mix up langauges
To summarise, the articles said that trying to learn two languages at once is hard, but if you're pretty comfortable with one, starting a new language won't affect it too much. Also the more different the new language is, the less it will interfere with the old language.
As Welsh and Spanish are very different, it shouldn't be too much of a problem. Now I just have to sort out my learning methods because SSiS on it's own is not working well for me. I know a lot of people have done really well with the method, but my memory isn't good enough and there are some words and phrases that are just not sticking. I'm thinking of trying flashcards for extra practice.