Suppose that the President of the United States nominated a candidate to two different judicial positions (say a district court judge as well as a circuit court judge) and also the Senate confirmed that person for both positions. Could the President then award a commission for both positions at the same time? Or is there a principle that prevents holding multiple offices in the Judicial branch (it is clear that a person can hold an office in the Executive and Judicial branch, see John Marshall who served as Secretary of State and Chief Justice.
Statutes provide that federal judgeships are a full time job. It isn't expressly constitutionally prohibited, however.
Indeed, there have been times in U.S. history when higher court judges served in lower federal courts "riding circuit" to meet personnel needs. This is still done by senior judges and sometimes in other circumstances, on a temporary, often case by case basis. Similarly, for example, all judges on the FISA Court are also federal court judges with other primary posts. This doesn't require a separate Presidential appointment.
In lower courts, for example, municipal courts, where the judgeship is a part-time position, it isn't uncommon for one person to be a judge in multiple courts.