Dual accreditation

Dual accreditation is the practice in diplomacy of a country granting two separate responsibilities to a single diplomat. One prominent form of dual accreditation is for a diplomat to serve as the ambassador to two countries concurrently. For example, Luxembourg's Ambassador to the United States is also the non-resident Ambassador to Canada and the non-resident Ambassador to Mexico.

Such an Ambassador may sometimes be called Ambassador-at-Large.

...
Wikipedia