Both of two things; indicates that each of the two items is included.
私はそのケーキを二つとも食べてしまった。
I ended up eating both of those cakes.
二つとも同じ色だから見分けがつかない。
They're both the same color, so I can't tell them apart.
この商品は二つとも保証が付いています。
Both of these products come with a warranty.