Combination Classes are what are called “Multi-classes” in other versions of D&D.
All combination classes of any 2 of Fighter, Thief, Cleric and Magic-User are permitted, as long as no other restrictions apply (no good-aligned cleric-thieves). The combined Experience Per Level has been charted out. Also, instead of rolling and averaging the hit dice, an averaged die is used (roll a d6 instead of averaging a d4 and d8, for example).
Experience Point Charts:
Document contains all of the Experience Point Charts for combinations of 2 classes: combination_class_charts.pdf