This question depends on the NAT (Port forwarding) in your router. Most routers just look at the port number, and then forward based on the port number, like this:
Pseudo code: anything.com:incomingport --> localIP:port
Example: *:80 --> 192.168.1.10:8080
In this example, any request that hits the router on the external IP using port 80 is sent to 192.168.1.10:8080 - it doesn't matter if the incoming request was "cam1.dyndns-server.com:80" or if the request was "cam2.dyndns-server.com:80", they would both end up going to 192.168.1.10:8080.
That being said, there's more elaborate routers that WILL actually look at the URL in the incoming request, and can use that information to determine where to send the request. I know that Microsoft Internet Security and Acceleration server does this, but configuration is more than what you'd probably want for a home. :-) If your router has that feature, then this is possible to accomplish. If your router does not have that feature, then I'd simply put them on separate external ports on your router, like the following. The only drawback is that you'll have to enter the port number when you access the webcams externally:
cam.dyndns-server.com:81 --> 192.168.1.10:80
cam.dyndns-server.com:82 --> 192.168.1.11:80
HTH