Is it possible ?

Scenario that worked:
New scenario (possible?):
Questions:
The new scenario seemed be possible according to sb4200 docus..
If then, wut about the dynamic IPs given by the ISP, do they give 2 IPs to a single HFC-MAC address ? Look like it will be only a single IP given ? Then how to config the usb connection to sb4200, the slackbox can access to Internet in this scenario ..

Let me see if I"m understanding you. (Your diagrams are a bit difficult to decipher)

You have a cable modem that can be connected via USB or by ehternet. You want to know if you can simply use both connections and have two boxes connected, each with their own IP address. (if this is not right please elaborate).

AFAIK, this is not possible. Unless you want to pay extra for another IP, you're only going to get one (dynamic) IP from your provider. Therefore you're going to need some sort of NAT/IP masquerading to connect multiple boxes (which is what your first diagram seems to be showing). Of course you could always purchase a router to share your connection too, but if you only have two boxes, then (again. I'm assuming) you've already done it with your original setup.

You have correct interpreted the diagrams. According to motorola surfboard sb4200 cable modem description, it should be possible to have both connection nic+usb through the modem to internet. I assume also that the ISP won't give extra IP for the same modem (HFC-MAC-ADDRESS), just gonna try it out.

I'd experienced few times that my slack server hung, when I started internet explorer on the sat2805 (connected to slack-server through cross-cable, 1st diagram), internet connection sharing(ICS) activated on slackServer (ala: iptables -t nat -A POSTROUTING -o $EXT_IF -j MASQUERADE; iptables -A FORWARD -i $INT_IF -j ACCEPT).

The only possible outway from the hang is to hardboot it and it caused many dirty partitions that had to be e2fschk-ed. The worst is that last time during my 2.6 compilation it hung while I started to browse using IE on sat2805. After reboot, the "/" partition is too dirty of the compilation harddisk activity. The kernel rebooted it self several times, and run e2fschk on that still not clean "/" partition. I finally reinstall slack-current to the partition. That is background for the advanture with multiconnection through sb4200 ..

Back to question:
I wanted to know whether the ISP-dhcpd will give another IP to my HFC-MAC address at all (seemed less possible), bcoz the ISP-dhcpd must had some maping HFC_MAC_ADDR:IP, so if the slackBox already got an IP from it, it'd know if that same MAC_ADDR is trying to request for another IP again ..

I tried to force broadcasting dhcpcd request on sat2805 (dhcpcd -t 15 -R -d ethX),
but I got no answer. Btw wut does "multicast" mean ?

Oh yeah wut is the device name for usb ? Looked like lspci shown that sat2805 got another nic (that from the cable modem) beside that builtin etherExpress ..