By default, PowerGUI Script Editor is a single instance application – all scripts get opened as different tabs on the same window, and all the scripts and interactive commands in the same PowerShell session. They “see” the same variables and so on.
This however is very easy to change. Just go to Tools / Options and clear the Allow only one instance of the application to run – and you can have as many PowerGUI Script Editor windows as you like.
Each window will have its own PowerShell runspace – totally isolated from other windows (but still shared within the tabs of the same window).