I don't know Haskell, but in Lisp, and IIRC also in ML, a "let" expression is shorthand for a lambda application, so:

    let x = 1 + 1
    in x * x

is equivalent to:

   (\x -> x * x) (1 + 1)

As the denotational semantics of any lambda expression is represented by its normal form, for function application that would be the result of evaluating that function with substitution; the normal expression is 4, which denotes the integer 4.

Similarly,

    let x = 1 + 1
        y = 1 + 1
    in x * y

is equivalent to:

    (\x -> \y -> x * y) (1 + 1) (1 + 1)

and, so, by the functional denotation, this also denotes the integer 4.

As 4 = 4, the denotation of these two expressions is equal.