On the Xbox 360 the HDMI output and the RCA output are quite close together and this often impedes the ability to have a HDMI cable and an RCA cable connected at the same time.
To be able to have both the HDMI and the RCA connections you use the following adapter: (http://www.bestbuy.com/site/eforcity-optical-rca-audio-adapter-and-hdmi-cable-3ft-bundle-for-xbox-360-slim/1305211122.p?id=mp1305211122&skuId=1305211122).