If you have a proportional Knob on your transmitter, then I would consider making that your throttle.
Then you have the two joysticks free to control the pitch on each drive. That will give you the most
control and maneuverability using the VS drives.
If you have one of those dual stick throttle transmitters just laying around, Like the F14 marine, then
The dual sticks would control the forward and reverse pitch of each drive, and then gang the steering, rudder,
stick to the steering of both drives. Throttle has it's own proportional channel. That is how I run my VS driven fire boat.
For Throttle, you can get away with a forward only ESC. As Pitch control on the VS drives gives you directional control.