With Plex if I want to stream my media from the server to two devices at the same time would I need plex plus or can I do that as a standard free user?
You do not need a Plex Pass subscription to stream to more than one device concurrently.
In fact, Plex not only lets you stream to multiple standard clients on your home network, but also lets you stream to your mobile phone (or even you laptop) outside your network, with no subscription required.
Saying that, an additional feature you can get (relating to this) is Plex Sync. This lets you easily download a video to your phone (in Plex) so you can watch them later without a network connection. That's a feature of Plex Pass.