You can choose, a set (6chips CYMK WW ), two chips(white cartrediges) , or 1 chip for individual white cartredige. 
6-chips:
* compatible Epson C13T72510N black ink cartridge
* compatible Epson C13T72520N cyan ink cartridge
* compatible Epson C13T72530N magenta ink cartridge
* compatible Epson C13T72540N yellow ink cartridge
* 2x compatible Epson C13T725A0N white ink cartridge (A1 and A2)
 
Or:
A1, A2
A1, A1
A2, A2
A1
A2
 
This note is only about the two white ink cartrediges;  
Most of the time, your printer will work with these cartridges with the default combination (A1; A2). Although it is not common, some printers have been known to reject one of these chips. In this case, they will  function with another combination, either (A1; A1), or (A2; A2).
If this is the first time you are using refillable cartridges, start with the default combination (A1; A2). If this does not work (which is rare), you can purchase an A1 chip or an A2 chip separately.