Creo Simulate > Reference Links > Combination Criteria for Load Histories:
  
Combination Criteria for Load Histories:
You can define different load histories for multiple load sets on the Fatigue Analysis Definition dialog box.
The following load history combinations are possible:
Any combination of constant amplitude histories.
Combinations of variable amplitude histories with the same number of steps or values in every load set.
Combinations of variable amplitude histories with the same number of steps combined with a user defined constant amplitude history. The minimum and maximum load factor for constant user defined amplitude in every load set must be equal.