[bende511]

I can speak to my experience as the child raised multilingually, though not quite the same situation you are proposing. I was raised in the US to a an American dad and a French mom, and they both speak both languages. My parents decided they would only speak to me and my younger brother in French, assuming (luckily correctly!) that I would learn English by simply being in an English-speaking world. My experience, and I think my parents and their multi-lingual friends would agree, is consistency. My parents even went so far as to pretend not to understand us if we spoke to them in English! Not even to ask for a word translation! If you speak English, you always speak English. If your spouse speaks Spanish, they always speak Spanish. My feeling as a recipient of this education is that its probably better if you both speak the same language to your children, but I know that splitting them up can also work. Again, the key is consistency, and the simpler you can make the rules about who speaks what language the better.

[BizarroLand]

That actually sounds like a fun game for multilingual people.

Partner A picks language A, Partner B picks language B.

Now, have a conversation with each other where you only speak in your chosen language. The listener must listen and translate in their head and then respond in their chosen language.

First person to slip up and change languages loses.