Final answer:
To increase screen space with dual monitors side-by-side, select 'Extend these displays' in the multiple display settings which creates a large continuous workspace.
Step-by-step explanation:
To increase the amount of screen space available when configuring dual monitors positioned side-by-side, you should set the multiple display option to Extend these displays. This option allows the desktop to span across both screens, creating one large continuous workspace. In comparison, the 'Duplicate these displays' option will mirror the same content on both screens, 'Show only on 1' will display content only on the first monitor, and 'Show only on 2' will display content only on the second monitor, none of which will increase the overall screen space.