External Thunderbolt drive and Monitor from same port?
Thunderbolt as a technology supports daisy-chaining devices (connecting one device into another). Unfortunately a number of the devices available like the 2.5" GoFlex adapter and the video converters don't have the second port to allow this.

Your option could be to get the 3.5" GoFlex adapter and mount your drive in that and connect the screen to the second port on that.

There are some HDMI splitters on the market but this will just send the same image to each screen.