QUICK ANSWERStart with the tool that matches your immediate input.
Open Spreadsheet Sheet Merger first when its stated purpose matches the result you need now. Use Spreadsheet Sheet Splitter only when it solves a separate next task.
This guide is for writers, office teams, students, and data workers. Start with Spreadsheet Sheet Merger when your immediate task is to use the Spreadsheet Sheet Merger to complete this file task privately in your browser. Move to Spreadsheet Sheet Splitter only when you also need to use the Spreadsheet Sheet Splitter to complete this file task privately in your browser.
The goal is not to run two tools automatically. It is to finish the first narrow task, inspect its result, and then decide whether Spreadsheet Sheet Splitter solves a genuinely different next step.
Both tools sit in Documents & Files, but they handle different inputs or outcomes. Keeping those roles separate reduces repeated work and makes verification easier.