i have 2 camera with dahuaddns.
120212.dahuaddns.com
21212.dahuaddns.com
but after i put the new camera, i'M always get the new one and the old one i cannot view.
if i will be on my local network i'm see the old one also.
Both of those resolve to the same single IP address assigned to your house. So regardless of which of those addresses you use, they will both hit the home router the same way.
You'll need to use two different port number overrides when access the two cameras; perhaps http://120212.dahuaddns.com:8080 and http://21212.dahuaddns.com:8181 then set two port forwarding rules; one that forwards incoming 8080 to X and the second forwards 8181 to Y.