[1.8.1] Parallel android devices - single server or one server per device?


I am a bit confused as to what approach to have regarding parallel runs on multiple devices on a single computer. I’ve read here https://github.com/appium/appium/blob/master/docs/en/advanced-concepts/parallel-tests.md#parallel-android-tests that I could have a single server configured to take control of multiple devices.
However, I had previously understood that I could also achieve this goal by having one appium server for each device. I have two android devices, and right now I am experiencing some errors when running the same command on the two devices at the same time (for example launching an app or finding elements).

Disclaimer: I must use Appium version 1.8.1 and cannot upgrade.

Thank you all for your time,
Felix Ganz