Appium 1.7.0+ supports single server and multiple sessions, and it requires Xcode 9 for iOS.

Example

Run tests on Multiple Simulators with Xcode 9

• https://github.com/appium/appium-xcuitest-driver/tree/master/test/functional/parallel
• https://github.com/appium/ruby_lib/tree/master/ios_tests/parallel
  • Thread programming: TestParallelRunThread
  • Process programming: TestParallelRunProcess