Why is multi thread downloading faster than single thread?
Usually this is because somewhere between you and the other server there's a firewall limiting each HTTP stream to 10Mbps. When you use multi-thread, you get 2x 10Mb (one for each thread).
This is due to your ping between you and the server and packet size/tcpip window size used by your downloading software.
Basically, if you have 100ms ping to the server, and request packets of 100kb, you can only get 10 packets per second using 1 connection, even if your internet speed is infinite.