I don't know of any specific reason they shouldn't be used together (concerta and dxm) but you should ask a doctor before trying such a combo. Of course most doctors would warn against exceeding the recommended dose of dxm with or without other drugs in your system. You might try abstaining from concerta for a week before using dxm, but again, you should ask a doctor first, because I dont know what the half life (how long it takes for half of the drug to be eliminated from the body) of concerta aka Methylphenidate is and because concerta is a timed-release capsule of methylphenidate, meaning that whatever its half life; it still takes longer to absorb than the non timed-release kind.