Each camera needs to have its own incoming port mapped to the camera IP address, you cannot map a range of ports to more than one device.
Camera 1 on port 80 would need its own rule.
Camera 2 on port 81 would need its own rule.
You would have to change the port number on the second IP camera to port 81. Then map port 81 incoming, to the IP address of the second camera.
To access the second camera from its web address, you would have to use http://xxxxxxx.xxx:81/ where xxxx is your DDNS host name.
The :81 specifies the alternative stream (port number).
Each camera must have its IP address set to static IP, outside of the home hubs DHCP range.
Typically 192.168.1.1 to 192.168.1.63
There are some examples on this page.
I think there are two issues here that is causing your confusion.
As an example I have four cameras connected to my BTHub via ONE STATIC IP ADDRESS and the app that I use on my Android phone can access them all at the one time via my DNS address.
( That would be the bit from the CCTV shop saying one port all cameras )
Presumably as you are using port 80 you are trying to access your CCTV box via HTTP and log into it; from there you will most likely get a menu that will allow you to select a camera.
If that's right then I see a problem; HTTP access isn't secure so if you have to send a password anyone will be able to see it.
Just my thoughts !!!
The second cams access port number would need to be changed from its default port 80, to port 81.
Most IP cameras will allow you to specify which port they provide their stream.
If its not possible to change it, then you will have to map port 80 port 80 for cam 1 IP address
Then translate port 81 incoming, to port 80 of cam 2 IP address.
The port will show as closed unless the mapping is correct, and the second camera is turned on.