You are wrong of course.

I tried two possibilities:

1.

Restore wallet using both master public key and I have this watch only wallet. And you will see in the watch only wallet - there are no active button to sign a transaction.



2.

In the 2nd possibility I did this:

 - Used master key for the first cosigner key
 - Used 2nd cosigner's seed for the 2nd cosigner

I was able to sign a transaction but if you closely monitor the restore process then you will see, I just opposed the order of imputing the cosigners information when I restored the wallet.
Instead of entering cosigners seed first, I entered the cosigners master public key. In both case (for the 2nd possibility), the algorithm is same which is actually restoring the same cosigner wallet which seeds are known to you only. Unless you know the seeds from the other cosigner, you can not broadcast the transaction.