Thanks for the contributions coconutty and soft top.
The PDAs already have seperate identities so thats not the problem......
I had considered seperate logons but felt that this way was a better route as I wanted to just leave the pc on for anyone in the family to use (the kids are too young for logons!) and I hoped that the next step of viewing and making appointments in both/each others calendars would be easier this way.
I've installed active sync 3.7.1 now but the problem seems to be still there, but after several attempts (rebooting solves the problem) I have narrowed it down to whether or not outlook is opened whilst the PDA is connected. Then once the PDA connection is severed and outlook closed (or is it?) re-launching outlook opens only the one profile.