Here is a list of metroplexes that overlap multiple countries:
Excluded from list is:City-states being part of the overlap. One example of that would be Rome since Vatican City is isolated within Italy.
Brazzaville, the capital of the Republic of the Congo, lies directly across the Congo River from Kinshasa, the capital of the Democratic Republic of the Congo
Goma, Democratic Republic of the Congo and Gisenyi, Rwanda
Livingstone, Zambia, and Victoria Falls, Zimbabwe
City of Lomé, the capital of Togo, extends over the border to Ghana
Aqaba, Jordan; Eilat, Israel; and Taba, Egypt
Tel Aviv, Israel metropolitan area covers an area of the West Bank that includes Israeli as well as Palestinian towns.
Jerusalem, Israel metropolitan area covers an area of the West Bank that includes Israeli as well as Palestinian towns.
Al Ain, UAE and Al Buraimi, Oman
Dibba Al-Fujairah,UAE and Dibba Al-Baya, Oman
Astara, Iran and Astara, Azerbaijan in coast of the Caspian sea
Vientiane, the capital of Laos, spills over the Thai–Lao Friendship Bridge (Saphan Mittaphap) to Nong Khai province, Thailand
Singapore together with Johor Bahru in Malaysia and Batam, Bintan, and Great Karimun in Indonesia, also called the Sijori Growth Triangle.
Hong Kong's metropolitan area spills over its border with Mainland China into Shenzhen and Dongguan.
Macau's urban sphere stretches across the border of China into Zhuhai.
Vienna-Bratislava metropolitan area overlaps Austria and Slovakia
Strasbourg-Ortenau Eurodistrict, France and Germany
San Sebastian metropolitan area, Spain and France
Meuse-Rhine Euroregion extends over Maastricht, Netherlands, Liège, Belgium and Aachen, Germany
Øresund Region extends over northeast Zealand (Copenhagen), Denmark and western part of Scania, Sweden
Upper Silesian metropolitan area extends from Katowice with Upper Silesian Metropolis, Poland to Ostrava, Czech Republic. This metropolitan area lies within Upper Silesian Coal Basin.
Eurometropolis Lille–Kortrijk–Tournai, France–Belgium
Luxembourg metropolitan area, extends into Belgium, France, and Germany
Frankfurt (Oder), Germany and Slubice, Poland are one city divided by the river Oder
Görlitz, Germany and Zgorzelec, Poland are one city divided by the river Nysa
Cieszyn in Southern Poland and Český Těšín in the Czech Republic are one city divided by the Olza River
Salzburg in Austria extends to Freilassing, Germany
Gorizia in Italy and Nova Gorica in Slovenia
Nice (France) metropolitan area extends from Grasse (France) to Ventimiglia (Italy), also including the Principality of Monaco.
Komárno in Slovakia and Komárom in Hungary
Valga in Estonia and Valka in Latvia are one city historically, divided in 1920 by national border
Saarbrücken metropolitan area, Germany extends into France ( Forbach, Sarreguemines ) .
Derry metropolitan area, Northern Ireland (UK) extends into County Donegal, Republic of Ireland
Tornio, Finland and Haparanda, Sweden
Nicosia, Cyprus metropolitan area extends from Republic of Cyprus to the unrecognised Turkish Northern Nicosia.
Algeciras and La Línea de la Concepción, Spain metropolitan area extends to the border with Gibraltar, United Kingdom
Tallinn, Estonia and Helsinki, Finland is a proposed metropolitan area across the Gulf of Finland (Talsinki)
Kerkrade and Herzogenrath, historic town divided by the Dutch German border, border passes along the middle of a street
Milan metropolitan area, Italy extends into Canton Ticino, Switzerland
Basel metropolitan area, Switzerland extends into France and Germany
Geneva-Lausanne metropolitan area, Switzerland extends into France
Iaşi metropolitan area, Romania extends over Prut, including cities like Ungheni from Republica Moldova
Lower Danube metropolitan area (Brăila-Galaţi (Cantemir) metropolitan area), Romania extends over Danube and Prut, including the Republic of Moldova's urban area of Cantemir-Cahul
Subotica, Serbia and Szeged, Hungary - urban sprawl between the two close cities
Slavonski Brod, Croatia and Brod, Bosnia and Herzegovina, divided by Sava river
Polish Tricity, Gdynia (a city built to become a port of the Second Polish Republic) and its neighbours Gdańsk and Sopot, overlapped two borders in the period 1919-1939:
In Free City of Gdańsk: Gdańsk and suburbs
In Poland: Gdynia
In Weimar Republic (later Nazi Germany): Sopot
List of metropolitan areas that overlap multiple countries Wikipedia
This list is from west to east:Sault Ste. Marie, Michigan and Sault Ste. Marie, Ontario
Detroit, Michigan and Windsor, Ontario — see also Detroit–Windsor
Port Huron, Michigan and Sarnia, Ontario areas
Buffalo-Niagara Falls metropolitan area
Ogdensburg-Massena, New York (St. Lawrence County) and Cornwall-Prescott, Ontario (U.S. micropolitan area and Canadian census agglomeration)
San Diego, California, and Tijuana, Baja California — see also San Diego–Tijuana
El Centro–Calexico, California, and Mexicali, Baja California — see also Calexico–Mexicali
Yuma, Arizona, and San Luis Río Colorado, Sonora
Nogales, Arizona, and Nogales, Sonora
Naco, Arizona, and Naco, Sonora
Douglas, Arizona, and Agua Prieta, Sonora
El Paso, Texas, and Juárez, Chihuahua — see also El Paso–Juárez
Del Rio, Texas, and Ciudad Acuña, Coahuila
Eagle Pass, Texas, and Piedras Negras, Coahuila
Laredo, Texas, and Nuevo Laredo, Tamaulipas — see also Laredo–Nuevo Laredo
McAllen, Texas, and Reynosa, Tamaulipas — see also Reynosa–McAllen Metropolitan Area
Brownsville–Harlingen, Texas, and Matamoros, Tamaulipas — see also Matamoros–Brownsville Metropolitan Area
Rivera, Uruguay and Santana do Livramento, Brazil.
Río Branco, Uruguay and Jaguarão, Brazil.
Chuy, Uruguay and Chuí, Brazil.
Ciudad del Este in Paraguay, Foz do Iguaçu in Brazil and Puerto Iguazu in Argentina, also called Triple Frontera or the Tri Border.
The Tres Fronteras metropolitan area of Tabatinga in Brazil, Leticia in Colombia, and Santa Rosa in Peru form a metropolitan area of nearly 100,000 people.
The Cúcuta-San Antonio del Táchira area which spans the Colombia-Venezuela border.
Ipiales, Colombia and Tulcán, Ecuador.
Chile Chico, Chile and Los Antiguos, Argentina