The campaign, called "Building Broadband Britain", kicks off later this month and hopes to take advantage of rival BT's poor start to its ADSL high-speed access offering for retail customers.
The two cable giants will offer broadband connections over cable for £24.99 a month compared with BT's £40 a month. The cable companies do not compete for customers because their networks pass different parts of the UK.
NTL and Telewest will face competition from BSkyB and ONdigital. However, cable has the advantage that it can carry traffic to and from the home over the same network. BSkyB and ONdigital can only carry traffic to the home, and relies on a telephone line to carry the signal upstream.
Both digital broadcasters are currently believed to be in talks with BT about an agreement. ONdigital is expected to announce a deal on Wednesday when it rebrands as ITV Digital.
Although NTL and Telewest are working more closely for the first time, they are believed to be resisting a merger for financial reasons. Both companies have spent millions of pounds building their networks and NTL, the bigger of the two, has £10bn in debts.
However, broadband is an area where the two companies can combine and compete against other networks. NTL managing director Stephen Carter said the joint venture marketing initiative was a "natural progression".
He added, "The case for cable has, over the years, been dominated by road-building and disruption. This is an opportunity where we can lead. When we came into telephony, we were number two to BT. In multichannel TV, we were number two to BSkyB. In broadband, we can be leader."