When u put ur router in bridge mode, it basically becomes a switch and a plain modem.
You will still have your LAN running but the WAN functions like doing its own PPP, NAT etc will fall away. So you will have to setup a PC to do the WAN stuff and use PPPOE.
It works basically the same as a 56k dialup connection in term of sharing and port fowarding.
If openweb only allows one concurrent connection, i dont think u can set two pc's to dial up. If this is the case, id suggest setting up one pc to dial in and share the connection, then set the other pc to use the one dialing in as a gateway. Then u just set your 3rd pc to dial into webafrica