I'd like to run two unipolar stepper motors at 6V 1.2A each and two geared dc motors at 9W 1.2A each simultaneously on the same motor controller. What options are there for this particular setup, and also, are there any benefits to splitting the setup into two controllers instead?
I would actually go with a L293 H-Bridge. Since its an H-Bridge Driver, it can control 2 motors per chip (one on each side of the chip) independently. You can also change the direction of the output current depending on input (also independent on both sides of the chip)
Just get 2 L293's, and use one of them to control the 6V stepper's and one of them to control the 7.5V (9W) DC's. It goes without saying that you need different circuits powering these. So if you want to use one power source to power both these circuits, you will want to look into using a mofset to turn a 7.5V current to a 6V current.
Also, you want to watch out that you only use the "bidirectional" functionality on your steppers since sending the opposite current to ur DC's could fry em.
Since you will use 4 inputs on the L293 controlling the stepper's, and 2 inputs controlling the DC's, you will use a total of 6 GPIO pins to send booleans to your chips. Furthermore, you will need a separate power supply (not from the Pi) since your motors will require far more than 5V @ 1.2 A to power. If using a circuit to the Pi and a separate one for the Motors, you only need 1 moffset. However, if you want to use one current to power both motors and Pi, you have to convert 7.5V @ 1.2A -> 6V @ 1.2A -> 5V @1.2A. Hence, you need 2 mofsets.
Benefits: It depends on what you want to accomplish. If you need one pair of motors to have the ability to rotate bidirectionally while leaving 2 only unidirectional, then there is a point. However, if you are just making a 4 wheeled vehicle, then this setup is a complete waste. Just get a chip that can send 4 outputs at once.
The common hobby motor driver boards only have one supply voltage. Unless you find differently you will need one or more driver board for the 6V steppers and a separate driver board for the 9V DC motors.
I suggest you look at L298N motor driver boards.
If your steppers need 4 inputs you will need a motor driver board per stepper.