If your 650 is like my 645, it can't be changed.
The tibias are wired like a pipe organ's unified rank where the exact same pipes are available on both the upper and lower manual. Since both manuals use the exact same rank of pipes, the volume on both will be the same.
In the Conn, the tibia stops for both manuals go through the exact same keyer circuits which select the tones before going to the amp. If it makes you feel any better, theater pipe organ players have the same problem.
So, at least with the Tibias, you are getting the faults of a unified theater organ along with the sound of one.