On your oscam server you need to have two accounts, one for each DM500HD box. You add/edit these accounts in the oscam.user file.

On each client (DM500HD) you only have to alter the newcamd.list file which is located in /var/keys or /usr/keys.
The newcamd.list file should look like this:

CWS_KEEPALIVE = 300
CWS = 192.168.0.100 portno username password 10 02 13 04 15 06 17 08 01 10 11 12 13 14

You have to substitute portno, username, password and the DES-key (10 02 13 04 15 06 17 08 01 10 11 12 13 14) for the actual values you have configured on your oscam server.