If you have set the "Session Mode" preference (Tools > Preferences > Options tab page) to "Multi Session", then each SQL Window, Test Window, Command Window, Report Window and tool Window will use its own separate session. All sessions will be for the same Oracle user though.
If you want to have multiple sessions for different users simultaneously, then you need to start a multiple PL/SQL Developer instances.
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Marco Kalter
Allround Automations