I did this last month with NTL cable modem and it works OK.
Before I proceed any further I would point out the following: -
(a) The Master computer (one actually connected to the modem) must be switched on, and not in hibernation.
(b) The Slave Computer will not be able to access the internet if the Master computer is not as in (a) above.
If you still have a problem if (a) and (b)are OK
then please post a reply.