In protocol design, why would you ever use 2 ports?

Question

When a TCP Server does a socket accept on a port, it gets a new socket to work with that Client.
The accepting socket remains valid for that port and can accept further

Answer 1

You should have a look at the RTSP + RTP protcol. It is a similar design : each stream can be sent on a different port, and statistics about jitter, reordering etc... is sent on yet another port.

Plus there is no connection since it is UDP. However it was developped when firewall where already something banal (sorry for my english), so a mode was developped where all this connection could be embedded in one TCP connection with HTTP syntax.

Guess what ? The multi port protocol is much simpler (IMO) to implement than the http multiplexed one and and it has a lot more features. If you are not concerned with firewall problem, why choose complicated multiplexing scheme when there is already one existing (TCP port)?