How is this different from a birthday attack ?

In Nx2 DES, you have A -k1-> B -k2-> C . In a birthday attack, you try values of k1' and k2' until you get k1', k2' s.t. E(A,k1') = B' = D(C,k2') but that's only true for that particular plaintext set A,C. If you have B' = the same value of B that the original k1,k2 produced, you win, but there may be many other values of B' besides the one for k1'=k1, k2'=k2.