I see no-one responded to your post yet, so I may not be the best resource. I had the same issue tho.
When both pc's connect via LAN cable you cannot run ICS I know that for a fact (the WHY i'll leave to the experts to explain). There doesn't seem to be a way around it.
What you can do is though is to connect on pc via USB, connect and then share that with ICS. But this means once your iburst is connected and you switch of that pc, your other pc do not have connectivity anymore. the iburst modem in itself cannot act as a router and connect itself, nor can you connect two pc's with the normal PPPOE simultaneously.
Your answer and probably your best option is to get a router. These routers most of them have multiple LAN ports as well. Then you connect your modem to the router via LAN, your pc's to the router via LAN and run your router web-interface on any of the pc's IE to set up your connection router level. (wow, hope that makes sense)
There are a number of routers ppl can probably suggest. I didn't wanna go to expensive and bought a Netgear 108 mbps wireless router (I got one laptop that now connects wirelessly and one pc via LAN cable) for about R800 or so. Model name WGT624. As I say, it may not be the best or anything, but it works for me right now. If I recall correctly, there are cheaper routers that doesn't offer the wireless lan (also Netgear) and goes for about R400 -R600.
Hope this helps.