The account became free in May this year when NatWest parted company with its direct marketing agency, TBWA\GGT. The move ended a relationship that had lasted for five years.
Craik Jones and Joshua will now share the business on a project basis.
The briefs will incorporate NatWest's personal banking, loans and insurance services.
Joshua handles the direct marketing for NatWest's business banking and savings products. The agency won the £5 million account at the beginning of the year.
Craik Jones is also on the roster of NatWest's parent company, the Royal Bank of Scotland, dealing specifically with its credit-card range.
The decision does not affect the bank's relationship with its above-the-line agency, M&C Saatchi, and its media agency, MediaCom.
TBWA\GGT originally won the account, as TBWA\GGT Simons Palmer, in the summer of 1998. At the time, the account incorporated all direct marketing, brand advertising and sales promotion and was said to be worth approximately £80 million.
The agency subsequently set up the "NatWest village" - an integrated internal team dedicated to the bank's advertising. This collapsed when the above-the-line account moved to M&C Saatchi in 2000.
As well as RBS, Craik Jones also has the Prudential Assurance account, and its financial credentials are bolstered by a spell on Barclays' direct marketing roster.
Joshua has a host of financial services companies on its client list.
The agency handles the through-the-line business for Zurich Financial Services as well as the Morgan Stanley, Norwich & Peterborough and HBOS corporate accounts. The agency also helped its stablemate Grey London land the Visa business in February 2003.
Earlier this year, RBS announced its intention to spend £150 million rebranding its 1,650 NatWest branches. This programme spelt the end for the company's famous black-and-white logo.