The example implementing the execution of concurrent Future async calls in dart synchronized by the Google's Tuple API, as described at https://stackoverflow.com/questions/42176092/dartlang-wait-more-than-one-future/67736970#67736970

dart-concurrent-futures-google-tuples-sync.dart

  // Fetch the ticket status with fee associated with it
  // 2 future calls are made concurrently, so that it will be constructing the status object properly
  static Future<TicketStatus> fetchTicketStatusWithFee({ required String uid, required int marketPlaceId,
    required int storeId, required String ticketId }) async {

      // https://stackoverflow.com/questions/42176092/dartlang-wait-more-than-one-future/67736970#67736970
      // Calling both endpoints and sychronizing on them
      final tupleTicketStatusAndFee = await waitConcurrently<TicketStatus, double>(
        // Future<TicketSTatus> is returned on item1
        fetchTicketStatus(uid: uid, marketPlaceId: marketPlaceId, storeId: storeId, ticketId: ticketId),
        // Future<double> is returned on item 2
        getServiceFee(uid: uid, marketPlaceId: marketPlaceId, storeId: storeId)
      );

      // Set the ticket status after getting it
      TicketStatus ticketStatus = tupleTicketStatusAndFee.item1;
      ticketStatus.fee = tupleTicketStatusAndFee.item2;
      return ticketStatus;
  }

dart-test.log

$ dart test test/parking_lot/ticket_status_remote_repository_existing_full_test.dart
00:02 +0: Given the ticket id that exists the call must return an exception
┌───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ #0   ParkingTicketsRemoteRepository.fetchTicketStatus (package:xxxxx_platform_client/cash/_super/platform/parking_lot/parking_ticket_remote_repository.dart:42:13)
│ #1   ParkingTicketsRemoteRepository.fetchTicketStatusWithFee (package:xxxxxxx_platform_client/cash/_super/platform/parking_lot/parking_ticket_remote_repository.dart:26:9)
├┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄
│ 🐛 Fetching the ticket status from xxxxx at https://xxxxxxxx/v2/parking_lot/tickets/327177824178
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
┌───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ #0   ParkingTicketsRemoteRepository.getServiceFee (package:xxxxx_platform_client/cash/_super/platform/parking_lot/parking_ticket_remote_repository.dart:74:13)
│ #1   ParkingTicketsRemoteRepository.fetchTicketStatusWithFee (package:xxxxx_platform_client/cash/_super/platform/parking_lot/parking_ticket_remote_repository.dart:27:9)
├┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄
│ 🐛 Fetching the payment service fee at https://xxxxxxx/v2/payment/servicefee
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

╔╣ Request ║ GET
║  https://xxxxxx/v2/parking_lot/tickets/327177824178
╚══════════════════════════════════════════════════════════════════════════════════════════╝
╔ Headers
╟ X-xxxx-Tid: a84c3c1c-78d2-4e23-80ad-94742ef56ab3
╟ X-xxxxx-Uid: 1234567
╟ X-xxxx-MarketplaceId: yyyyy
╟ X-xxxx-App-Version: 1.0
╟ X-xxxxx-StoreId: 1
╟ followRedirects: true
╟ connectTimeout: 5000
╟ receiveTimeout: 3000
╚══════════════════════════════════════════════════════════════════════════════════════════╝

╔╣ Request ║ GET
║  https://xxxxxxx/v2/parking_lot/payment/servicefee
╚══════════════════════════════════════════════════════════════════════════════════════════╝
╔ Headers
╟ X-xxxxxx-Tid: e4373e14-352e-48c1-8642-e778e8f20b20
╟ X-xxxxx-Uid: yy
╟ X-xxxxxx-MarketplaceId: yyy
╟ X-xxxxx-App-Version: 1.0
╟ X-xxxxx-StoreId: 1
╟ followRedirects: true
╟ connectTimeout: 5000
╟ receiveTimeout: 3000
╚══════════════════════════════════════════════════════════════════════════════════════════╝

╔╣ Response ║ GET ║ Status: 200
║  https://xxxxxxxx/v2/parking_lot/payment/servicefee
╚══════════════════════════════════════════════════════════════════════════════════════════╝
╔ Headers
╟ connection: [close]
╟ date: [Sat, 17 Jul 2021 06:35:07 GMT]
╟ vary: [Origin, Access-Control-Request-Method, Access-Control-Request-Headers]
╟ x-xxxxxxx-api-version: [v2]
╟ content-type: [application/json]
╟ x-xxxxxxx-build-version: [3c5cf42e-develop]
╟ server: [nginx/1.19.6]
╟ x-b3-traceid: [c8bb963ab935f6a2]
╚══════════════════════════════════════════════════════════════════════════════════════════╝
╔ Body
║
║    {
║         service_fee: 80
║    }
║
╚══════════════════════════════════════════════════════════════════════════════════════════╝
┌───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ #0   ParkingTicketsRemoteRepository.getServiceFee (package:xxxxx_platform_client/cash/_super/platform/parking_lot/parking_ticket_remote_repository.dart:95:13)
│ #1   <asynchronous suspension>
├┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄
│ 🐛 service fee: 0.8
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────

╔╣ Response ║ GET ║ Status: 200
║  https://api.super.cash/v2/parking_lot/tickets/327177824178
╚══════════════════════════════════════════════════════════════════════════════════════════╝
╔ Headers
╟ connection: [close]
╟ date: [Sat, 17 Jul 2021 06:35:08 GMT]
╟ vary: [Origin, Access-Control-Request-Method, Access-Control-Request-Headers]
╟ x-xxxxxx-api-version: [v2]
╟ content-type: [application/json]
╟ x-xxxx-build-version: [3c5cf42e-develop]
╟ server: [nginx/1.19.6]
╟ x-b3-traceid: [8160099eae9492b2]
╚══════════════════════════════════════════════════════════════════════════════════════════╝
╔ Body
║
║    {
║         status: {
║             cnpjGaragem: "12.200.135/0001-80",
║             dataConsulta: 1626503708310,
║             dataDeEntrada: 1626468337000,
║             dataPermitidaSaida: 1626504300000,
║             dataPermitidaSaidaUltimoPagamento: null,
║             errorCode: 0,
║             garagem: "xxxx SHOPPING",
║             idGaragem: 1,
║             idPromocao: null,
║             imagemLink: null,
║             mensagem: "",
║             notas: [
║             ],
║             numeroTicket: "327177824178",
║             promocaoAtingida: false,
║             promocoesDisponiveis: false,
║             setor: "ESTACIONAMENTO",
║             tarifa: 1700,
║             tarifaPaga: 0,
║             tarifaSemDesconto: 1700,
║             ticketValido: true,
║             valorDesconto: 0
║        }
║         ticketStatus: "NOT_PAID"
║    }
║
╚══════════════════════════════════════════════════════════════════════════════════════════╝
┌───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
│ #0   main.<anonymous closure> (file:///Users/marcellodesales/dev/gitlab.com/xxxxxx/clients/xxxxx-platform-client-dart/test/parking_lot/ticket_status_remote_repository_existing_full_test.dart:62:14)
│ #1   <asynchronous suspension>
├┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄┄
│ 🐛 Ticket Status for ticketId=327177824178: TicketStatus: [ticketId: 327177824178]
└───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
00:03 +1: All tests passed!