you are just depending on the level area where you are getting xp from . in the top right corner of the screen you can see the level area where you are located . the character/both characters need to be + or - around 10 levels (in my experience it worked up to 13 levels difference) to the area they are located in order to gain xp. if the character level is too high or too low compared to the area , it won't gain any important xp , just around 8 xp per kill for normal and nightmare , which is not enough to gain any quick level-ups .
so to answer your question , about 20-25 levels difference between characters , while the area level must be somewhere in the middle until the lower level character gains more xp .
also , if your primary character is already very high lvl compared to the secondary character , the primary can help the other one to power level in areas close to the secondary character's level , while the primary won't get any important xp there , just the secondary