iif_defer.ts

// API Service
export class ApiService {
    get(parameters: SomeParameters): Observable<ReturnType> {
        if (!this.validate(parameters)) {
            // throw error on failed validation
            throw new Error('Parameters do not pass validation');
        }

        return this.httpClient.get(...);
    }

    getOther(parameters: SomeParameters): Observable<ReturnType> {
        if (!this.validate(parameters)) {
            // throw error on failed validation
            throw new Error('Parameters do not pass validation');
        }

        return this.httpClient.get(...);
    }
}

// SomeOtherService
export class SomeOtherService {
      someFlag: boolean;
      
      constructor(private apiService: ApiService) {}
      
      getSome(someParameters: SomeParameters): Observable<Some> {
            return iif(() => this.someFlag, this.apiService.get(someParameters), this.apiService.getOther(someParameters));
      }
}