Just need to multiply the maximum current of your load (the motors, 2400mA) by the number of motors (2). So it's 4800mA or 4.8 amps. Which is a pretty beefy power supply - the biggest plug-in type ones I've seen are about 2 amps.

Remember that the adafruit motorshield is only rated to drive 0.6 amp (600mA) per motor too (though you can upgrade http://www.adafruit.com/forums/viewtopic.php?f=31&p=123944) and that'd require a 1200mA supply, though in practice this isn't necessary. I have been including a 1000mA supply in my kits.

In general, low voltage motors will _usually_ needs high current to provide the grunt to get things moving, and a low current motor _usually_ needs higher voltage. I in no way resemble an expert, but I think of voltage vs amperage as being like speed vs torque: There's a relationship between the two but to get a certain power output they are inversely proportional to one another. Voltage isn't that significant in stepper motors either (http://www.schmalzhaus.com/EasyDriver/) and the shield can handle 30v+ so there's no need to keep it low.

sn

I still have these 2v 2amp motors sitting around here, but I fried a couple of motorshield chips and a power supply with them - learned the hard way!