You can't run these off the Arduino board for several reasons:
- the Arduino board does not provide 7.4V on any pin, only 5V (model dependent, but never 7.4 V)
- the Arduino board can provide about 20mA per pin, never 1A or higher
- even if the board would only need to pass the voltage on, the total current were >2A. This would possibly fry the board.
So, what you need is: a power supply with constant 7.4V or a regulated one where 7.4V is within the range. From this power supply you will power 3 devices only:
- one arduino board via the Vin pin (accepts 7 to 12 V, so 7.4V is ok)
- two Dual Channel DC Motor Drivers (accepts 4 to 16V, so 7.4V is ok)
From each motor driver, you will power
- one Planetary Gear Unipolar Stepper Motors (designed for 7.4V)
Now, let's sum of the currents:
- Arduino Uno board without any peripherals: ~45 mA
- two Stepper motors: 1000 mA each
- two motor drivers: hard to say. In theory, the motor driver would not consume power by itself, just forward it to the motors. In practice, it cannot forward everything. The driver will get hot and thus consume some power itself. The datasheet is not specific about the power consumption. Let's assume a 70% efficiency = 30% loss. 30% of 1000mA is 300mA each.
The total current needed for this setup is 2645mA. Depending on your budget you could buy a 3A or 5A power supply. At constant 7.4V, that's 23W or 37W. For a regulated 0V to 20V power supply, go for 60W or 100W.
A cheap regulated one should be at ~50 USD. Expect those to be loud because of bad cooling / fan noise.