From mstsc.exe /?:

/multimon -- Configures the Remote Desktop Services session monitor layout to be identical to the current client-side configuration.

So to have two monitors in your RDP session while using this switch, you'll need two monitors set up on the RDP server (the VM in this case).

To add a second monitor to a VirtualBox VM:

• Ensure the VirtualBox Guest Additions are installed in the guest OS.
• Power down the VM.
• Right-click the VM and choose "Settings..."
• Choose "Display" in the left-hand panel.
• Turn up the "Monitor Count" to 2 (you may need to assign more virtual Video Memory for each monitor you add).

From mstsc.exe /?:

/multimon -- Configures the Remote Desktop Services session monitor layout to be identical to the current client-side configuration.

So to have two monitors in your RDP session while using this switch, you'll need two monitors set up on the RDP server (the VM in this case).

Edit:

User noted (after answer was posted) their VirutalBox tag was wrong, and that they're actually using Hyper-V as the hypervisor.

There is (currently) no way to add a second virtual display to a Hyper-V VM; So you're stuck using the mstsc /span option to utilize multiple monitors on the RDP client.

Orginal Answer (VirtualBox):

To add a second monitor to a VirtualBox VM:

• Ensure the VirtualBox Guest Additions are installed in the guest OS.
• Power down the VM.
• Right-click the VM and choose "Settings..."
• Choose "Display" in the left-hand panel.
• Turn up the "Monitor Count" to 2 (you may need to assign more virtual Video Memory for each monitor you add).