What you want to do right now is not something you can just configure as far as I know.
The same goes for the 7.XX Lotus Notes Client. The best thing to do would be to wait for Lotus 8.
Lotus 8 offers the best of both worlds. You get the Sametime 8 (which is pretty much like 7.5 ST ST Client) and give you the nice new 8 Notes Client as well.
I faced this question daily after deploying the 7.5 ST Client and at times wished I never had installed it anywhere. While you can be logged in to ST from the Client and Notes at the same time you can’t give on more “weight” than the other.
If anyone else knows a way to pull this off then let me know as it would help plenty of people.