In the VDI / SBC space this is refered to true multimonitor. This is a limitation of RDP. RDP only spans displays. It does not offer a true multimonitor experiance like a workstation. There are several third party tools you can add to the virtual desktops to acheive this. On th software side tou can use
Splitview or iShadow
On the hardware side you can use WYSE multimonitor capable thin clients with their TCX software.
WP