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It can be done, but you may have to use your powers of persuasion on ntl sales as it's not a standard request so they need to look further than their normal screens.
You will need to have the accounts in different names, of course satisfying the ntl rules that both account holders are independent and are in fact separate households and both over 18.
It might be an easier and cheaper solution to upgrade the account to the (up to) 10 meg speed, and introduce a router so you both share the one connection.
Edit: beaten to it