"Side by side" refers to two things that are positioned next to one another in a parallel manner. Some synonyms for this phrase that you could use to add variety in your writing include "alongside," "adjacent," "parallel," "neighboring," "abreast," "parallelly," and "in tandem." Depending on the tone, context, and style of your writing, you could also use descriptive phrases such as "shoulder to shoulder," "hand in hand," "cheek by jowl," "face to face," "hip to hip," "heart to heart," or "eye to eye." These synonyms can help to create a visual image in the reader's mind while also avoiding repetition and providing a fresh perspective on the concept of things being positioned next to each other.