What is the meaning behind the titles of Tokyo Ghoul OVAs JACK & PINTO? For reference, the dictionary definition of the word "pinto" is 


  Marked with spots of white and other colors; mottled; spotted.


It can also be used to refer to a piebald horse which has irregular patches of two colors such as this one:
My guess is that "Jack" is a reference to "Jack the Ripper" because in the OVA, the main characters are chasing a serial killer. It could also be a reference to "Jack-o'-lantern" since the serial killer wears a pumpkin mask and is referred to as "Lantern". On the other hand, the only two things I can come up with for Pinto is that it's a reference to (spoilers ahead): 
1 


   The hidden bruises on the elderly patient which look like the spots of a piebald horse. The patient's skin was thus like a "pinto" horse.



2  


   The dual personalities that the nurse had because "pinto" could mean the she had spots of white (good) and black (bad).



I think theory #1 on the elderly man is more likely to be right than the nurse theory.