If you're going to go for a recreational dose of DXM you should not take your opiate dose in the same day, if possible. If that would cause you withdrawals then make sure you only take a small dose, enough to keep you out of withdrawals, then wait at least a few hours to dose the DXM.
It all depends on your dosages but normally large doses of DXM and opiates are going to put you at risk for dangerous CNS depression.