The phrase "occurring at once" is correct and usable in written English.
It is used to indicate that something happened immediately, all at the same time, or simultaneously. For example: "The house was engulfed in flames, occurring at once and spreading quickly throughout the structure."
The two occurring at once is obviously far more rare than either of them happening.
What sort of urban landscape is it that has such things occurring at once, and what sort of places are these that are so astonishingly disparate in asking price?
Haiti's devastating earthquake in January 2010 created an unprecedented orthopaedic catastrophe, with hundreds of thousands of serious skeletal injuries occurring at once, even as the infrastructure for caring for them was simultaneously wiped away.
Even to speak of the transition in the singular is, in a way, misleading, given that there are many changes occurring at once: the handover of institutions from one set of hands to another; a businessman becoming President; an electorate witnessing a season of bitter campaigning give way to a period of governance.
Bulley wrote a generative sound score, composed into six general states, each of which were triggered by a set of conditions occurring at once.
Recent multivariate analyses can cluster voxels in the brain or SNPs on the genome to empower analyses, sometimes with both forms of clustering occurring at once (Hibar et al. 2011a; Thompson et al. 2013).
During cardiac surgery a large amount of cerebral microemboli can occur at once and create so-called "cerebral embolic showers".
Without a strong mental model strategy can become open to interpretation, decision making can become bogged down, or both can occur at once.
Harvey, he said, is a prime example of how these two factors work together to create the perfect storm, producing catastrophic coastal flooding when they occur at once.
Great occurrences like the Napoleonic invasion happen not because one man dictates the movement of history, but because hundreds of thousands of motives and accidents and reactions occur at once; Tolstoy called this the "swarmlike life, where man inevitably fulfils the laws prescribed for him".
However, these changes did not all occur at once.
