If your modem can't tell the difference between your two computers, then neither can EvC.
It works for everything else, somehow.
Does the DSL modem connect to the Internet or WAN input of your router, and then your two computers connect to LAN ports? If so, then why do you fake a MAC address? The router should take care of IP mapping for you.
I had to "activate" my DSL modem from one of our computers (hers, as it turns out) because my ISP doesn't want connection sharing without paying a stupid fee, and I wasn't able to get it activated through my router's firmware options. So the DSL modem won't pass data to anything but her computer's MAC address. So the router has to spoof her MAC in order to connect.
After that, the router handles NAT and the IP space, and like I said, it works for everything but this, apparently.
What the hell. Let me try to disable spoofing and we'll see what happens.