Rumor has it, that hidden deep, deep within the sunny pastures of Petaluma, there is an implementation of BXXP written for Microsoft Office. Less well hidden, there are two implementations of BXXP written in Tcl. We'll answer these in reverse. Errr, actually BXXP does let TCP do the hard work. In practice, there are no deployed application protocols that do this.
Some sites deploy load-balancing devices that transparently rewrite IP addresses between servers.
What this means is that if you open up two TCP connections to the same IP address, it's anyone's guess as to whether they actually terminate at the same physical machine. You could argue that HTTP does this successfully, and you'd be half-right. Browsers that use HTTP often open up multiple connections to the same servers, but from the server perspective each connection is coming from an independent entity, and the server still has to do all the authorization stuff on each connection. However, the reason BXXP does multiplexing is because many application protocols can make good use of parallelism. A Quite the reverse.