In the context of generative music, the term 'combined aleatory' describes methods used to generate music aleatorically, or by chance. An analog way is to record multiple tape loops of similar sounds playing back simultaneously, each loop a different length so they will gradually offset from each other. An example of this can be heard in Brian Eno's "2/1" from Music For Airports.
Aleatory in the digital realm can be achieved by triggering layered samples through a computer algorithm, or based on parameters such as duration, density, and attack and release times of notes. I recently recorded a track this way using Drone FX, a generative ambient music standalone and iPad app. It allows for a range to be selected on each parameter, so when the next event is triggered the value of the parameter will fall somewhere in between, creating the aleatory but also allowing for some control of the composition.