I took two 10mg ritalins (methylphenidate) about 4 and a half hours ago, can i take a 5mg valium to help me sleep? or will these two not mix?

Not a good idea: Stimulants and benzodiazepines ( such as valium) are generally not used together - though there can be exceptions. Please discuss with your prescriber.

You can but: Although you can but these are both addictive medicines and have to be prescribed by your physician for a particular diagnosis.Sometimes they are prescribed together.But you should discuss their use with your psychaitrist or treating physician, as it can cause addiction.