As already mentioned in comments - DOCSIS cable modems don't talk to each other, they need CMTS for communication & provisioning system to make them work.
If you are serious about playing with DOCSIS and have some money to invest in this you can build working lab these days relatively in-expensively.
CMTS - search on Ebay or ask refurb hardware sellers, you are looking for something like Arris C3, Cisco uBR 7200-series (note it is not regular 7200, it must be uBR) or Motorola (BSR1000 or 2000, but I never used those).
Provisioning system, which is used to:
- issue DHCP leases to CMs & CPEs behind your CMs
- generate and issue config file to CM
- provide other services such is time sync
- commercial ones have lots more bolted on top
You can build it from open source components. At the minimum you need DHCP, TFTP, ToD (Time of Day) and Syslog daemons and this software to create your DOCSIS config files. Alternatively you could ask one of the vendors for trial (time and/or number of CM limited).
You can find (not very up-to-date, but still partially valid) list of vendors on docsis.org
At last, but not least... DOCSIS is very popular and DOCSIS admins are very desired... just in case you don't know :)
Best of luck!