Works Now: If I set the laptop monitor to disabled first, before trying to enable twinview (but didn't apply any setting changes). Then enable twinview on both monitors (a warning was output, but nvidia-settings didn't crash and can currently use both external monitors).
Works Now: If I set the laptop monitor to disabled first, before trying to enable twinview (but didn't apply any setting changes). Then enable twinview on both monitors (a warning was output, but nvidia-settings didn't crash and can currently use both external monitors).