Two bowls backing into each other still may share one local outlet and do not alone prove a main sewer blockage.
Food particles, grease, scale, a failed disposal, and pipe geometry are different explanations. Avoid declaring the cause before the line is opened or inspected. The repair goal is stable drainage, not simply making one sink empty once.
Keep this boundary: Pouring very hot water or more chemicals into a standing clog can create burn and chemical exposure risks without locating the restriction.