Attack on a key-exchange,symmetric-key cryptography protocol
I'm not sure but in my point of view.

Since $k_{AB}$ is used to authenticate both side.
Mapped $nounce_x:E_{k_{AB}}$$(nonce_x∥k_x)$ pair on one side can be used on both side.
Thus we can gather $nounce_x:E_{k_{AB}}$$(nonce_x∥k_x)$ pairs from $B$ since $B$ will happily echo $E_{k_{AB}}$$(nonce_x∥k_x)$ for every $nounce_x$ I asked.

This doesn't form practical MITM attack but I can make $A$ agree on $randomK_1⊕k_2$ as $k_s$ while on the other side $B$ will accept $k_1⊕k_2$ as $k_s$. In another word we can make $A$ and $B$ agree on different values at probability equivalent to (or more than because we can query it ourself) birthday attack.

This might make some cryptanalysis possible but I don't know the details.

As you suggested in chat:
$nonce$ collision is unnecessary we rather change the scheme to something like:
After received $nonce_A$ we send it to $B$ twice but only sent the first response to $A$