De Guia de serviços do TravelMate 6592 / 6592G (PDF):
To enable the dual monitor feature of the notebook, first ensure that the second monitor is connected, then select Start, Control Panel, Display and click on Settings. Select the secondary monitor (2) icon in the display box and then click the check box Extend my windows desktop onto this monitor. Finally, click Apply to confirm the new settings and click OK to complete the process.
Espero que, nesta tela, você veja três opções de exibição (do seu laptop e dos dois monitores externos).