Hey Gooner44,
You mentioned it is fullscreen on the second client when server play is enabled. Have you made any changes is your Level Editor - Play settings?
To find these settings click the dropdown arrow next to the ‘Play’ button, and choose the ‘Advanced Settings.’ This will bring up the options I mentioned which contain window sizes and other controls when playing in the editor.
Advanced Play Options
Let me know if changing these settings resolves your issue, or if you need more help.
Cheers,