That's because you can't do that under JAA regulations!
The Seneca is a single pilot aircraft so only one pilot
can log pilot in command under JAA.
If you are both FAA qualified then you could log it in accordance with 91.109 but the safety pilot time cannot be counted as PIC for JAA.
In your scenario only you clould log the flight time for JAA purposes.
As a JAA CPL holder you should know this! Basic knoweldge for a commerial pilot.
Appendix A page 44 of
LASORS 2008 | Personnel Licensing | Safety Regulation applies.
This had also been covered many times before! Use the search function or look two posts above. In pretty sure this post will me merged with the logging of flight time thread or moved by a moderator shortly.