Altimeter disagreement. What's the crew procedure to deal with that?
Since you describe a captain and FO, I'm going to assume that you are asking about a larger aircraft, like say an airliner or business jet.

Part of the certification of these types of aircraft require them to have a third (standby) altimeter in addition to the two primary altimeters.

When there is a discrepancy, the typical procedure (keep in mind that a pilot should always follow the manufacturers checklist in these situations) is to cross-check the three altimeters and find the two that agree so that the bad one is then excluded.