[arksingrad]

MITM can only re-send the correct key if he knows the correct basis to measure in for every qubbit. The probability that he measures in the correct basis for every qubit is exponentially unlikely as the length of the bitstring grows. He can't just forward along the proper qubit to the receiver in this case.

[gliptic]

But he has man-in-the-middle'd the channel over which that is communicated too. If the benefit is merely that he would need to hijack two different channels, you could just do classical crypto and splitting the key into two parts (e.g. XOR with random bits) and send those over two channels.