Welcome to the community, Grace! :grinning_face_with_big_eyes: Unfortunately this is not currently possible directly within Airtable. Airtable fields can only be one type. They’re either a manual entry field where you type in text, numbers, dates, etc., or they are calculated based on something else (a formula, rollup, lookup, etc.). A field can’t be both manually editable and calculated.
That said, you may be able do what you want with the help of third-party integration tools like Zapier or Integromat, though the crossover might not be immediate depending on which one you choose and whether or not they support instant updates. You might also hit a circular update issue. For example, you change A, so the integration service is triggered and updates B to match. But then than might in turn trigger an update from B back to A, which might trigger an update from A back to B, and so on. I haven’t tried this kind of cross-table match update myself, so there may be ways around this that I’m not aware of.
