Folks,
Although it may vary on different types, the simple theoretical answer is that the REAR brakes on any given bogey get hotter for two related reasons;
1) The heat from the front brakes radiates to the rear, adding to the heat caused by the rear's normal braking action.
2) The front tyres and wheel assemblies block airflow from cooling the rear's brakes.
Actual relative brake temperature on any given aircraft type will vary depending on ambient conditions, length of taxi, brake usage, bogey design, brake wear and condition, tyre creep etc etc etc.