Imo mixing opioids and benzos is always a bad idea, as both are CNS depressants and can lead to respiratory depression. While 10 mg diazepam isn't much 350 mg codeine is (I guess you have a tolerance?). As this is a harm reduction forum pretty much everyone will tell you to not do it as it's not safe and not worth risking your life.
If you insist on combining both take really low doses of both and slowly work your way up. And only do it if you have someone you trust around who can give you some naloxone in case of an overdose and can call an ambulance (as naloxone has a rather short half life it's possible for you to slip back into the OD once it wears off, so calling an ambulance is still mandatory)