Hello TGSI assume that a knight always tells the truth and a knave always lies.

So let be the proposition "A is a knight", and be the proposition "B is a knight"

Then , since A says "Both of us are knights"

Therefore

Therefore B tells the truth

Therefore , since B says "A is a knave".

Contradiction.

Therefore is false, and A is therefore a knave.

Now B always lies

Therefore , since B says "A is a knave"

Contradiction.

Therefore is false. Therefore B is a knight.

Grandad