If you are using remote desktop and have multiple monitors on your client computer (the computer at which you are sitting in front of) - you can utilize all monitors for your remote session.
First, open Remote Desktop Connection
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Second, select "Show Options" at the bottom left of the window
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Third, check the box for "Use all my monitors for the remote session"
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Lastly, you can either go back to the General tab or press the Connect button.