I don't feel its illegal to share a BB. one of ma friend has airtel BB and the person from airtel made two separate points for him when he asked for sharing. Ofcourse he didnt pay anything extra.
There are two ways to do the sharing.
1) Buy a 3 port hub , connect 2 PC's and the "line" to this hub. configure the IP settings accordingly and you're done (may not work in some cases)
2)Get an extra NIC (LAN card) and then enable ICS on it. You may need to use NAT too.
PS: open up "help and support" and search for NAT and ICS, explining them will be out of scope of this post.