In earlier versions of XenApp you could make a desktop full screen in the published application settings. In XenApp/XenDesktop 7.5 this setting is no longer available. It's a little bit of an annoyance to have to click the top menu of Desktop Viewer to make it full screen, but even then it won't stretch across both screens in a dual monitor setup.
Lal Mohan at Citrixology has a very good post on how to edit StoreFront to launch a desktop in full screen mode.
On your StoreFront server, open the C:\inetpub\wwwroot\Citrix\<StoreName>\App_Data\default.ica file in Notepad, and add the line "DesktopViewer-ForceFullScreenStartup=true" under the [Application] section.