Why are some taxiways named with two characters?
Coding the taxiways this way makes it easy to create mnemonics. Note that from northwest to southeast, the first letters are alphabetically increasing. From southwest to northeast the numbers are increasing. 

The combination of a letter and a number gives a good indication of the location of the taxiway. 

Every taxiway close to the northern runway (5L-23R) is named Nx, everything close the southern runway (5R-23L) is called Sx. Numbering is from southwest to northeast, so N1 is close to the northern runway, at the west end (5L); S10 is the southern runway close to the eastern end (23L). 

Letters can also be combined:

The apron side of the N taxiway is where you park, and called NP. 

You cross on the east of the terminal on the eastern crossing or EC, or you cross on the west side of the terminal on WC or W2.