The published desktops in XenApp interpret two displays on a computer as one display, as shown in the following screen shot:
To resolve this issue the published desktop should be configured to launch in full screen mode.
To configure the published desktop to launch in full screen mode, complete the following steps:
In the Citrix Delivery Services Console under the Applications node, locate the target published desktop.
Right-click on the target published desktop, and select Application Properties.
Select the Appearance node in the Application Properties dialog box.
Select Full Screen for the Session Window Size.
Click OK.
Create a new session.
After logging on to the published desktop, right-click the desktop and select Screen Resolution. The server should display 2 monitors.
This error may occur because a published desktop only identifies the second monitor when configured to launch in full screen mode on the initial launch.