The companies have begun buying cable modems together and are in talks to order shared, low-price digital set-top boxes.
They have appointed Ian West, the former head of entertainment at BSkyB, to negotiate with the satellite TV operator for carriage of its Sky-branded premium movie and sports channels on their cable networks.
Sky has been accused of charging its rivals high prices to license its Sky-branded premium movie and sports channels. NTL and Telewest frequently complained, while West was at Sky, that they found it difficult to sell the channels at a decent margin because of the prices Sky was charging.
Sky is currently being investigated by the Office of Fair Trading about the prices it charges rival for its channels, and could be fined if it is found to have abused its dominant position in the market.
This latest announcement from the cable companies is part of a long-term agreement the companies struck earlier in the year to work more closely in an effort to cut costs, boost efficiency and stave off competition from rival telecoms companies and pay-TV operators.
In July, the companies launched a £4m advertising and marketing campaign to jointly promote the take-up of broadband internet access over cable in the UK.
Earlier this month, they signed an agreement to jointly produce a digital broadband TV service, which is due to launch next year.
However, a merger of the two companies is still thought to be some way off because of their large debt mountains -- Telewest's stands at £4.9bn and NTL's is about £11bn -- and differences in their accounting structures.
Telewest chief executive Adam Singer described the companies' position as being like a farmer's cooperative in France.
He said, "We are interested in buying the same tractors and selling the same grapes, but we remain our own companies."
There is also a difference in the services the companies offer. NTL is venturing into mobile services, while Telewest is concentrating on developing its fast PC internet subscription services, a move that has been received well in the City.
Telewest's shares rose 7.7% this morning to 80.75p in London. Yesterday, they rocketed 58p on the back of the company's third-quarter results, which saw earnings before tax, depreciation and amortisation increase 12% to £83m.
Shares in NTL, which trades on the New York Stock Exchange, were unchanged at $3.72.
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