Yes, you can unlock LFS on two computers with a single license. You can play with a friend on the second computer but only across a LAN - only one copy can go online at any one time. If you try to get the second PC online with the same account you'll get an error message.
yes, i have LFS on both my pro-rig (my lappy with a Saitek wheel), and my away-rig (works computer with a mouse and lots of outlook express and quotes for shiny new volvos getting in the way.)