Concomitancy is a fairly uncommon word that refers to the coexistence or occurrence of events or circumstances together. If you're looking for synonyms to describe a similar concept, there are a few different options you could consider. "Concomitance" is a very close synonym that has the same basic meaning. "Coterminous" and "coexistent" could also be used to describe things that occur at the same time or in conjunction with each other. "Simultaneous" is another option that could be used to describe things happening at the same time or in a closely connected way. Overall, there are many different words you could use to describe the idea of things happening together or in sequence.