Answer
This is possible in principle with a Select-statement custom field, but first confirm both source values actually exist as columns on the screen's own base view — it's a common trap to assume a value visible on screen is available directly in the underlying view when it may actually be pulled in through a separate join or API call the screen makes internally. If one of the two values isn't in the base view (as can happen with certain planning/analysis screens), your custom field's select statement needs to independently reference whatever other table or view actually holds that value, rather than assuming it can pull both values from the same base view the screen itself uses.