Coextensively is a term used to describe something that exists or occurs at the same time or in the same place as something else. Some synonyms for coextensively include simultaneously, parallelly, in tandem, and concurrently. Simultaneously implies that two or more things are occurring at the exact same time. Parallelly describes two or more things that are happening at the same time but not necessarily in connection with one another. In tandem implies that two or more things are happening in a coordinated, synchronized manner. Concurrently refers to two or more things happening at the same time in a shared space or context. Ultimately, these synonyms all describe the concept of two or more things existing or occurring together.