Both may occur together in the same example, depending on the relevant point of view.
Or had their night, and day together occurred to form some bond between them?
They used these to determine how often the two conditions occurred together.
These two types have different characteristics, and may occur together.
And this did answer his question about appointments that occurred too close together.
It may be best for this to occur in smaller groups or all together.
The two conditions often occur together and are both more common in women.
The two fish occur together and are known to hybridize.
"It wasn't just a coincidence that these three things were occurring together."