I grew up speaking English and Spanish, and now speak five languages fluently. I can easily think in English and Spanish, and can translate fluently enough in the other three to appear as though I can think in those languages. I favored English because it was the language spoken in school, with my friends, and my sports teams.
I believe the only downside to growing up speaking two languages is the grammar. My grammar in English is terrible (my writing kind of shows it) because only half my attention was on English, with the other half on Spanish.
Today, I only think in English, but Spanish is still spoken in my household and if needed I can use that too.