According to the rules of syllogisms you can't get a conclusion from two negative premises, but since an O proposition i.e. Some A is not B can be expressed as a I proposition, i.e. Some A is non-B, then isn't this latter form just as much an O as an I?
The syllogism you propose is quite valid, but it is not one intended under the rule you state, because you have in fact changed the minor term from C to non-C and the minor premise from a negative form to a positive one. This is legitimate, no problem.
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