latcismon, as a fellow wigginer I'll try and clear up your query.
Firstly, please be much more specific.
Are you using a diablo cam in the DM7020 ? If so, then no you can't share that card. Secondly, why are you using a Cam module ?
Simply put your card directly in the card reader of the DM7020, do away with the plugin CAM , install CCcam in both boxes and share the card to the DM500. Its dead simple.
In the DM7020 the CCcam.cfg would look like this
SERVER LISTEN PORT : 12000
F: DM500 Password 3 1 1 {0:0:2}
in the DM500 the CCcam.cfg would look like this
C: 192.168.100.1 DM500 Password yes
# Where 192.168.100.1 is the IP address of the DM7020 on your lan.
location of CCcam.cfg file is /var/etc
Its that simple. 2 lines in the server box, 1 line in the client box.
since this is all lan based there's no need for any port forwarding or DNS addresses.
Hope this helps.
B16MCC - Fellow Wigginer.