Multiple Instances and Availability Groups
This can happen due to the TCP ports configuration of your instances and listeners


  If one of the instances of SQL Server is already listening on TCP port
  1433 via the instance listener and there are no other services
  (including additional instances of SQL Server) on the computer
  listening on port 1433, this will not cause a port conflict with the
  availability group listener. This is because the availability group
  listener can share the same TCP port inside the same service process.
  However multiple instances of SQL Server (side-by-side) should not be
  configured to listen on the same port.


Source

In short, you can have both your listeners listening on port 1433, as long as you change the tcp ports for the instances to different ports like 1533, 1633.

You could also use different port numbers than 1433 but then you need to specify these in the connection string(s).

In the case of "one instance, one listener" you could be using the same port for both.