Yup. If you are running one cable, the expense of running two to the same location, in materials, is about...err, possibly $15-20.
You now have double the possible traffic speed between the two locations.
Typically heavy streaming is only going to hit you for probably 10-15% of your bandwidth. So even if you only have a 1Gbps link, you are still talking getting 100-110MB/sec for the file transfers with 1080p video streaming supposing that the traffic is going in the SAME direction.
If the traffic is going in opposite directions, there is no impact (other than ACK packets), as the links are full duplex, 1Gbps each direction. Traffic on the other side of the highway doesn't impact you on your side of the highway.
If the streaming is occurring from the same device that you are doing file transfers from/to (IE streaming from the NAS while also transferring files from/to the NAS), you are going to probably be looking more at disk limitations, memory footprint issues or a combo of both, more than you will be network link issues. Well, at least since I see you mentioned you have a NAS, which are generally not performance beasts.