Split 5V 3A into 1A in 2A to make USB charging device
You have a fundamental misunderstanding here, that is why there are so few results.  

You do not need to split anything. No, it will not be more efficient to split it into a 5V 1A port and a 5V 2A port.  Why would that be more efficient? And I don't understand what you think you're splitting.  Loads (like a phone or tablet) draw varying amounts of current, and will have a maximum current draw rating.  They will never draw more than this, but will draw much less (like when fully charged, for example).  The device will draw however much current it needs, and beyond that, it needs 5V.  You can't limit current using resistors because the loads aren't linear.  Resistors limit current by reducing the voltage, but a phone or tablet will simply stop using the port if the voltage goes outside the USB spec.  

When you see a USB port rated at 1A, that means it can provide UP TO 1A. You need an integrated circuit to shut off the port using a MOSFET switch in the event of over draw. You need active circuitry if you want to limit the current draw from the port, you cannot do it using resistors.  

But, there is no reason to do this.  You have one 5V power supply that can provide 3A.  Specify the two ports as having a combined maximum current of 3A, and connect them in parallel.  You're done.  

Now, USB ports cannot provide more than 500mA of current unless you signal the device it can draw more.  Any phone or tablet that conforms to the USB spec will refuse to draw more than 500mA from either of your ports, no matter what you do.  

You have to use resistors to make voltage dividers that yield specific voltages across the D+ and D- USB pins, and depending on which you chose, will signal different devices a certain kind of charger port is available. Unfortunately, many tablets use 2.1A, not 2A, so to call a 2A port a tablet port is not really correct.  It's insufficient for a great many tablets, while all smart phones expect a 1A charging port minimum.  So you're 100mA shy of having the capacity for a true phone and tablet charging device.  It might charge YOUR tablet, and if that is all you want, then no worries.  But if you plan to have other people use this, you need a slightly beefier 5V regulator.

As for signaling the maximum capacity for each port... well, you can do that using resistors, but high resistance, low wattage ones.  They create a voltage divider with the D+ and D- pins of the USB port connected in the middle.  However, there is no standard voltage value you want on the D+ and D- pins, it's not part of the USB spec.  So Apple generally has its own specific resistor values that are different from android devices, and Samsung has its own thing for its tablets... so its impossible to create a true universal charging port using resistors alone.  You have to use a smart port IC, which will detect the type of device connected to a USB port and set the voltages on D+ and D- appropriately.  The TPS2546 is one such IC.

If you only plan to use this for yourself, and only need it to work with your specific devices, then you can just use a resistor divider.  There is one for 1A Apple devices, then that same divider but with the D+ and D- voltages swapped yields a port for charging 2A apple devices.  There is one for sony devices, and one for samsung devices.  Then European devices all need only a 200 ohm resistor shorting D+ and D-, by law.  

Here is an example of what I mean: 

I don't know what device(s) you want the ports to work with, but if you simply google the voltages for a given device and current, you can find the information you need.  I also highly recommend you read this app note, which is a nice quick and dirty crash course on all this.  

Unfortunately, this is one of those situations where something that ought to be simple... is not.  C'est la vie.