Hi H.,
we are raising our kids English/German (they are almost 3 years old and 8 months old). We have many friends that are bi- and trilingual. We have read 4 books on this. We have lived in the US and in Austria since our children were born. My husband and i are both fluent in both languages, with my husband's German being a trifle weaker than my English (due to the age at which he learnt it and his limited first hand experience). So that is our background.
Here is what we do: I speak my mothertongue to both children (German). My husband speaks his mothertongue (English). When we were living in the US, we spoke German at home, when all of us were together (so called "family language"). This was meant to provide some balance to an overwhelmingly English speaking environment. Now we live in Austria (where German is spoken), and we have switched our family language to English, for the same reason.
As far as i understand and experience, and as the research i read about has shown, there are only a few rules about raising kids bilingually, but these are important.
And the first rule is: stick with one language. Don't alternate what you speak to your child, when you are speaking one on one. If the primary caregiver mixes languages for no reason apparent to the child, a kid can end up knowing no language fully at all.
Next rule: Choose a consistent family language.
Next rule: Don't mix what language you speak within sentences or paragraphs.
Next rule: Don't be afraid if your child only speaks mothertongue (whatever the primary caregiver speaks) at first. With different stages of life (daycare, Kindergarten, moving to other countries), the other language(s) will easily be activated and may easily become a primary tongue later.
Last rule: If you speak to your child in a tongue that is not your mothertongue you need a good reason. A language is not only words, but comes with a culture. That culture belongs to the language. Which is why we celebrate German, Austrian and American holidays at home. It's ok if you speak a non-mothertongue to your child if you also embrace that culture. It helps very much if you find others who will speak that language (in your case Spanish) as a mothertongue to spend time with your kid. Also you should spend time in a spanish-speaking environment, if you can. Books, songs, games and films in both languages are really helpful in our experience.
Our son (the older child) speaks mostly German. After we spent a week with his English speaking grandparents, his English got kick-started. He now mostly speaks English with my husband, and German with others. When we are together as a family, he will make jokes about "how papa speaks" and "how mami speaks", and he is able to say the same thing both ways. He may even know there are still more languages (asked me to read a french book we have in french), but maybe he was just being a parrot and repeating something he heard us say. Hard to know what's in his head! We didn't expect him to talk so well and in both languages yet, but of course we are happy. It could also have been that he wouldn't have spoken English until much later. Also, he will probably only be able to really differentiate languages at the age of four.
A friend of mine raises her daughter "in spanish" though she is Austrian. She does this, because the girl has an Argentinian father (divorced, lives there, not in Austria like mom and daughter), and because she is absolutely literate in latin American culture, and also knows the language very well. The girl speaks to her mom in Spanish, but to all else in german. The mom is the primary caregiver, but she and her daughter live with their Germanspeaking mother/grandmother.
Oh, sorry, this is probably more than you wanted to know. It's just so nice to "talk" to others about this. Literature is just taking off on this. I mean, young parents today are not the pioneers in bilingual education, but not far from it!
good luck!
D.