I still don't fully get it. So this can never translate to "Both cats are white, too"? Mesela "Onun bir kuşu, iki kedisi var. Kuşu beyaz. Kediler(i+n)in ikisi de beyaz." Bu yanlış mı?
Edit: Changed "Kuş(u+n)u" to "Kuşu" because in "his bird is white", the bird is not an accusative object as I thought at first.
I am sorry - of course - the bird is course the subject in this construction, not an object... Even if it may cause people to wonder, I will update my original question to include correct grammar. I mixed up Possessive + genitive ending in the cats with possessive + accusative (which is just incorrect) for the bird
İki means two and -si is the possessive ending (of). Onların ikisi - two of them
Kedilerin ikisi - two of the cats
Kedilerin ikisi de - Both of the cats
As you see "two of the cats" and "both of the cats" are a bit different phrases. The latter has more emphasis and we express that emphasis by adding a de .
If you want to say just both or the two you should again use them with possessive endings.
ikisi - the two (of them)
ikisi de - both (of them)
@ trashboat9 What @ Selcen_Ozturk means is that you need the word "OF" for possessive. That is "both OF the cats"; "two OF the cats"; "all OF the cats"; "some OF the cats" etc. Like the "color OF your hair" = "your hair's color". You wrote "kediler ikisi de beyaz?" means "The two cats both are white". Where is the "OF". Strange, that in this English sentence, an English speaker would unconsciously insert commas to make it understandable. That is: "The two cats, both, are white." Too bad this is not the best translation of the Turkish sentence given.
see also comment of Marie_goforit above.
iki+personal ending = both, so "onların ikisi" = both of them, "bizim ikimiz" = both of us, "sizin ikiniz" = both of you
"de/da" in this case means "also" or "too", and in Turkish, "de/da" is used a bit more often / deliberately than in English.