How to run multiple simulators on Xcode 9?

I solved my problem, it's "very simple" :]

On the simulator, you must choose a new device from Hardware > Devices. The new instance will starts.

Then, choose your model on Xcode.

Here is the solution.

Xcode allows you to run simulations in parallel (that can communicate with each other as well, if this is part of app functionality), but you cannot run parallel simulations on the same simulated device type (ie, you can't simulate on two iPhone 11 Pros, but your simulations could be, for instance, iPhone 11 Pro Max & iPhone 11 Pro.)

1. In Xcode, select your first simulation destination platform (Xcode-Product-Destination-select iOS simulator) and run & start a simulation (ie, ⌘B, ⌘R).
2. In Xcode, do Debug-Detach from <your_app__name>. Your app simulation will continue to run, but Xcode will no longer be connected to it.
3. Select a different iOS simulator device.
4. Run & start the second simulation (ie, ⌘B, ⌘R).

I suspect you could detach the second simulation from Xcode also by repeating step 2, and then reattach Xcode to the first instance, for instance if you want the ability to inspect what is happening on either simulated device, but I haven't tried this.