I use QT 5.9.1 on windows10 with an intel graphics card. Using one output sink is no problem.
Only adding multiple GstGLVideoItems doesn’t work. 
 
I add the qmlqlsink to my qml file as follows:
GstGLVideoItem 
            {
                id: idVideo0
                objectName: "videoItem0"
                x: 0
                y: 0
                width: parent.width / 2
                height: parent.height / 2
                Component.onCompleted: _AppMgr.vid0 = idVideo0
            }
            GstGLVideoItem 
            {
                id: idVideo1
                objectName: "videoItem1"
                x: parent.width / 2
                y: 0
                width: parent.width / 2
                height: parent.height / 2
                Component.onCompleted: _AppMgr.vid1 = idVideo1
            }
 
In C++ backend I wait till the vid0 and vid1 properties are set and then I start building the pipline.
Vid0 works as expected…. If I use the same pipeline with vid1 I get:
 
0:00:01.930173963  4088    cb23330 INFO               glcontext gstglcontext.c:1231:gst_gl_context_create_thread:<glcontextwgl0> Attempting to create opengl context. user chosen api(s) (any), compiled api support (opengl opengl3) display api (opengl)
0:00:01.941723030  4088    cb23330 INFO               glcontext gstglcontext.c:1242:gst_gl_context_create_thread:<glcontextwgl0> created context
0:00:01.941934098  4088    cb23330 INFO               glcontext gstglcontext.c:1258:gst_gl_context_create_thread:<glcontextwgl0> available GL APIs: opengl
0:00:01.959707209  4088    cb23330 INFO               glcontext gstglcontext.c:1071:_create_context_info:<glcontextwgl0> GL_VERSION: 4.5.0 - Build 22.20.16.4771
0:00:01.959846536  4088    cb23330 INFO               glcontext gstglcontext.c:1074:_create_context_info:<glcontextwgl0> GL_SHADING_LANGUAGE_VERSION: 4.50 - Build 22.20.16.4771
0:00:01.959894111  4088    cb23330 INFO               glcontext gstglcontext.c:1076:_create_context_info:<glcontextwgl0> GL_VENDOR: Intel
0:00:01.959931869  4088    cb23330 INFO               glcontext gstglcontext.c:1078:_create_context_info:<glcontextwgl0> GL_RENDERER: Intel(R) HD Graphics 530
……..
Error: Could not initialize window system