If the Portuguese economy was booming relative to Spain, then "the market" (investors) can still take advantage of that by investing in other Portuguese assets

If the Portuguese economy was booming relative to Spain, then "the market" (investors) can still take advantage of that by investing in other Portuguese assets such stocks and real estate. If a government wants to address a trade imbalance then import tariffs is one way to do it - or policy changes affecting cost of goods produced for export.