I think the check on IP addresses is how it's going to work to verify you aren't account-stacking. Dish will never know your modem's MAC address, as that is known only to your provider's head end. But every IP packet has to have a source and destination address and port. Most people have home routers which is doing NAT, so all your receivers will appear to be coming from the same IP.
The guy with 2 IPs will of course give them grief and probably trigger the audit police.
True enough, but you will find that giving your neighbor Internet service is against your ISP's TOS. Dish may be relying on your ISP to enforce it's own policies. And of course if you think about it, you could also run a telephone wire to your neighbor's house and get around the Dish audit police while account stacking to your heart's content. Dish is relying on the telephone company to prevent that too.