Controller Configuration: Check the controller settings in Steam for both sessions. Make sure that the controller is only configured for the session where you want it to be active. You might need to disable any shared controller settings.

Big Picture Mode Settings: Sometimes, Big Picture Mode can take control of input devices in unexpected ways. In the Steam settings, try disabling "Use Big Picture Mode" for the session where you don't want the controller to interfere.

Session Isolation: Ensure that Aster is properly isolating the sessions. You may want to revisit the configuration settings for Aster to confirm that input devices are being correctly assigned to each session.

Steam Input Per-Game Setting: For games launched through Steam, check if you can set the input method to "Forced Off" or "Disabled" for the second session.

Community Forums: Since this seems to be a specific issue with Aster and Steam, you might find others who have faced similar challenges in community forums or on platforms like Reddit.

Update Software: Ensure that both Aster and Steam are up to date, as updates may include fixes for such issues.