It doesn't work quite like this.
What happens is that as soon as the conditions are right to engage both motors, then it will. This is steady speeds, mid-torque above ~35mph. In electric mode it changes into this mode quite quickly, it seems to take a little longer if the engine is running.
If the bigger MGB were to throw in its full power then it'd push MGA backwards, the two have to be torque-balanced to both deliver useful power into the differential epicyclic set that combines them.
This means you only get twice the lower power motor, MGA, which happens to be 55kW. So at full power in CD2, which is only momentarily, you'd get twice the MGA's 55kW = 110kW. But in this case the gearing is against you so what actually happens is the clutches change and lock out MGA and all the power comes from the 110kW MGB motor.
If the engine is running with both motors engaged, the same thing happens: in this configuration the motor and MGA add together, both being 55kW, so giving 110kW total. MGB is, in this mode, essentially idling and only produces reaction torque (and rotational velocity, which allows the engine speed to vary) on the straight-and-level to stop it spinning. When more power is needed, MGB will apply additional torque to counter the additional torque of the combined MGA + engine, but only up to 110kW. In the Cadillac ELR, it will actually allow an exceedance of 110kW by adding in the MGB motor power too, up to the torque limit to balance across the epicyclic. But Volt/Ampera has been designed to deliver 110kW in all situations, hence the MGA+engine are each a half of the maximum power rating.
MGB also acts as the regenerator when MGA+engine are running, so if for example you are going downhill with the engine running in CS2, MGA would still be generating power from the engine, whilst MGB would then be regenerating what is left from the actual descent of the hill.