Hey all right! That is a very nice project, wish'd the source code was posted but overall it is a simple code to make. The technique of being able to operate 2 LEDs from 2 pins is called "Charlie Plexing", there is a ton of stuff online for that.
Essentially you are making all your outputs digital and on the pair of pins you are using for your leds, making the first side 0 and the other side 1 you light up one of the leds. By reversing you pins logic by making the first side 1 and other side 0 it will light up the other led. I works well with very few components cuz the pic can only source 25mA per pin so there is no need to limit your leds(hazer: I know we have argued this before but the proof is in the pudding).
The code is a simple loop that will toggle the correct pins with delays in the middle, easy peasy, it would be a great place start with programming micros.
EDIT:just saw this in my travels, check out tip#2. it uses a similar configuration to the one above, but is different and will make for different effects.
http://ww1.microchip.com/downloads/en/DeviceDoc/chapter%201.pdfAlso to make it slower is going to be very difficult with just a .hex code that is provided, however making the LEDs fade may be possible with an RC Snubber circuit, but will require at least a capacitor for every pin.
