Two SQL Server with one TCP port
With TCP, you can only have one process listening on each IP/port pair.

This applies to SQL Server.  The first instance you start binds itself to listen on your_ip:1433, then when you start the second, the OS will not allocate the socket to the the new instance of SQL Server, because it has already allocated that listener socket to your first SQL Server instance.  If you look in your error logs, you will see a "socket already in use" error of some sort when you start up the second instance.

If you have multiple physical or logical NICs you should be able to bind the SQL Server instances to the same port on different IP addresses.