It is a bit like when you have been married for a long time, ' says Susan Williams with just a hint of embarrassment.
When you have worked together for 15 years, no matter how different you may be, you start to think - and even act - alike.
So it was when the 'two Sues' - Sue Osborn and Susan Williams - had psychometric tests for their new jobs as joint heads of the newly created National Patient Safety Agency. They were alarmed to discover they were starting to think alike.
For five years Ms Williams has been job-sharing the post of chief executive of Barking and Havering health authority. Soon she - and her other professional half, Ms Osborn - will take family-friendly working to the top.
The NPSA will run a national reporting system for logging all mistakes, adverse incidents and near-misses in an attempt to ensure that lessons are learnt across the NHS.
The concept is probably just as radical as the working arrangements of those who will lead it.
While the notion of familyfriendly working is widely spoken of within the NHS, the reality is that senior job shares are few and far between. 'It is still a terribly unusual way of working, ' admits Ms Osborn.
So how did the job sharing come about in the first place?
'Sue [Williams] phoned me up one evening with the idea while I was in the bath having a gin and tonic, ' grins Ms Osborn. 'I was doing a unit administrator post in Camberwell doing normal chief executive hours: 70 hours a week.
Sue had just been on maternity leave from a unit administrator post in the same health district.'
'I saw my son and fell in love with him, ' says Ms Williams. 'I couldn't think about leaving him.'
That was in 1986, when general manager posts were being introduced. They put their idea to their manager. He 'bought' it, and they began to job share as joint unit general manager with Camberwell HA. Since then they have have managed Lambeth Southwark and Lewisham family practitioner committee before it became a family health services authority, led a project reviewing acute services in three London HAs, and then, in 1994, became joint chief executives of Merton Sutton and Wandsworth FHSA. They moved to Barking and Havering HA in April 1995.
Both women are married with two children each: Ms Williams's are 10 and 13; Ms Osborn's are 16 and 13. There have been times they have wondered whether to carry on working jointly, but both are resolute that a full-time job is not for them.
The only contractual agreement is that if one of them should get run over by a bus, the other is offered the job full-time.
The two admit that a lot of effort goes into the smooth running of such a senior job share: 'We phone each other every evening, ' says Ms Osborn. 'It is often for as long as an hour-and-a-half. You have got to work at making it work.'
They will certainly be working hard in the next few months.With the HA about to set up primary care trusts, both are ratcheting up their normal three days a week each. Ms Osborn is going to go full-time for the next three months, while Ms Williams will do four days a week to cover the vast workload ahead of them.
Once at the NPSA, their remit will include computer systems which alert GPs to interactions between commonly prescribed drugs, safety procedures within hospitals to avoid mistakes being made in operating procedures, and persuading drug companies to package their products so that drugs are not wrongly identified.
But if the system is to work, it has to have the confidence of staff.
Both have talked to clinicians about how it might work and have been pleasantly surprised to find that doctors are as keen to tackle the problem as they are.
'There will obviously be occasions where things go seriously wrong, where individuals are subject to criminal proceedings, ' says Ms Williams. 'But we can do an awful lot to help clinical teams develop ways of working that will minimise errors.'
They will be setting up a confidential phoneline to enable staff to report errors. Their governmentprescribed priorities include eliminating suicides by hanging within psychiatric wards.
But what about the cost of such measures? There is a bid in to the Treasury for funding, says Ms Williams, who will not reveal figures.
'It is very much back of an envelope. We are very aware, because of our current role, of the pressures trusts are under. It would be very difficult if trusts had to invest and there were not the resources to support it.'
Ms Osborn swings the conversation onto a more positive note: 'It is a very brave thing to have done this. It is the first in the world, an attempt to change the culture.'
And changing the culture, judging from their experience of jobsharing, is something these two women are very good at.