The two words seem to be very similar in sound and meaning.
Cadê tu? = Where are you?
Где ты(Gdye tï)? = Where are you?
Now the Portuguese wiktionary says (without citing any sources) that cadê is a
And according to Terence Wade in Russian Etymological Dictionary, где is related to Common Slavic root *-de as in здесь(zdes', here).Forma abreviada de "que é (feito) de?"
So apparently, if we believe the Portuguese Wiktionary, they are unrelated.
So is this just a surprising coincidence?