The duo are to launch an agency selection and performance management company, along with former PricewaterhouseCoopers accountant Liam Ferguson, who will provide financial business expertise.
The company, which launches in the summer, has a working title of 'E-Mission' and will be based in Manchester.
It will be both agency- and client-facing, with Bagnall taking charge of helping agencies evaluate their own performance, eliminate overservicing and measure their productivity.
The company will also monitor how agencies perform for their clients, and help in-house brands to source and select PR agencies.
'So many clients are not sure if their PR agency is any good and complain about the amount of money they pay them,' said Carroll. 'Similarly, PR agencies complain about lack of direction from clients.'
He intends to improve communication between the two and will advise clients on how to set criteria to measure an agency's success.
E-Mission will also aim to help agencies to win businesses by providing feedback on their creativity and strategic approach to pitches.
'A lot of agencies are poor at pitches and some clients don't know how to write a brief,' said Carroll. 'We will help agencies to improve their communication with prospective clients.'