How to chain two network requests

RequestChainable.swift

typealias RequestChainable<T: Codable> = (request: URLRequestBuilder, model: T.Type)

extension NetworkService {
    func chain<T, U>(_ r: RequestChainable<T>, _ r2: RequestChainable<U>, completion: @escaping (Result<(T?, U?)>) -> Void) {

        self.execute(r.request, model: r.model) { res in
            switch res {
            case .success(let value): completion(Result.success((value, nil)))
            case .failure:
                self.execute(r2.request, model: r2.model, completion: { result in
                    switch result {
                    case .success(let value): completion(Result.success((nil, value)))
                    case .failure(let error): completion(Result.failure(error))
                    }
                })
            }
        }
    }
    
      func chainx<T>(_ reqs: RequestChainable<T>..., completion: @escaping (Result<(T)>) -> Void) {
        var index = 0
        func doReq(index: Int) {
            let current = reqs[index]
            self.execute(current.request, model: current.model) { result in
                switch result {
                case .success:
                    doReq(index: index + 1)
                case .failure(let error ): completion(.failure(error))
                }
            }
        }
        
        doReq(index: index)
}

class Test {
    func test() {
        
        let r1: RequestChainable = (request: UserAPIs.getProfile, model: Token.self)
        let r2: RequestChainable = (request: UserAPIs.getProfile, model: Token.self)
        
        let service = NetworkService()
        
        service.chain(r1, r2) { result in
            switch result {
            case .success:  break
            case .failure: break
            }
        }
        
    }
}