Hello! I'm going to try and help you out here with what i've noticed.
so lets say for example [Thunderstrike] deals lighting (or shadow depending if you have the rune equipped) which means it has a chance to put an ailment of that type on the target. next it will also apply your next highest spell damage type on the target that is not the same damage type as the spell you casted.
You can only apply two at a time if we're assuming you have [Grievous Afflictions] from the Cabalist passive tree that allows you to put one other ailment on the target.
now lets assume your next highest spell damage is rend (from a random ring or gem that adds rend to spells). you cast thunderstrike few times, you get lighting and rend damage ailments going. you can then cast a different spell such as [Consuming Embers] which will apply the fire ailment + the rend ailment now you would have lighting, rend and fire ailments on the target.all you have to do cast different type spells to apply one different ailment but that second one will tend to be the same ailment applied unless you get [Immortal Offering] which fucks it all up on what your second highest spell damage is but you seem to overall do more damage but it's much less controlled way of applying a secondary ailment.

I hope this helps if you need anything else let me know (Haus.3829) is my tag