If you can see a preview of the Elgato during recording in Game Capture HD you can use a window capture with a sub region to stream it. I used to do this with my Hauppauge before I got an Elgato because the Hauppauge is not yet supported. Downsides of this are that the recording will be in HD but the preview is usually not shown in the same quality and thus looks poor. I don't believe that you can record using Game Capture HD while streaming with OBS directly because both cannot use the Elgato at the same time. Only 99% sure though sorry.