If you want both computers on at the same time then using two modems will not work because they cannot both be connected to the telephone line at the same time. There are two alternatives, one is to use a router and the other is to use Internet Connection Sharing. The ICS involves setting up a network between the computers and simply (i'm let to believe) enable the ICS on the host machine. A router option is more expensive but allows Internet access on either machine without a host machine needing to be on. The also involves creating a network, but with the router effectively acting as a server.
Microfilters are just small adapters that fit on the phone socket. Broadband interfers with voice calls on the telephone, and the filters effectively filter out the interference so voice calls are not affected.
Hope this makes sense and is useful.