Yes, the VCS only manages a single MotorMix.
It is possible to have multiple MotorMix (or equivalent) connected at the same time. Both can be used to control the VCS, and both will mirror changes to the MotorMix that you make on the VCS GUI. However, if you move a fader on one MotorMix it will not cause the other MotorMix(s) fader(s) to move until you change snapshots, or adjust that fader from the VCS GUI.