Connecting to the same Locally Logged In Desktop Session, from an RDP Client is an obvious challenge in any Linux Environment. But this should be the way, how RDP Protocol should behave as we find with Windows Machines.
Linux RDP Implementations like XRDP, will create separate Session with Remote Connections, rather than connecting to a locally available Desktop Session.
However you could use a combination of both XRDP and X11VNC utilities, to achieve the desired behavior.
A sample implementation with Debian10 LXQt Desktop (using LightDM as the display-manager) is given here;
fig. A Debian 10 Virtual Machine, being accessed simultaneously from an RDP Client (Remmina) and a native viewer (Virt-Viewer), using the same Session.
