This has been discussed many times before, but 354mg of DXM is way too much to potentiate the codeine and combining the two would probably cause a lot of nausea and wouldn't be fun at all, could possibly be dangerous. As a general rule, combing opiates with anything higher than medical doses of DXM is a bad idea.
I'm going to close this because there's no harm reduction in this combination, please PM me if you have any questions. If you're looking for information on potentiating your codeine, I recommend checking out this thread and this resource.