Portugal only, or Portugal plus Spain?
Keep Portugal as its own trip unless you have enough time to make Iberia breathe. Lisbon, Porto, the Douro, the Algarve and the islands already give more than enough; adding Spain should be a route choice, not proof that you used the map efficiently.
Answer
Portugal and Spain pair well on paper, but the border is not a magic shortcut. A rushed Lisbon–Porto–Madrid–Barcelona plan can become a greatest-hits exercise where every city loses its edges. Portugal works best when you give it room: mornings in cafés, coastal train rides, wine country, tiled streets, seafood lunches and a slower shift between north and south. Add Spain when there is a natural line — for example a longer Iberian trip or a route built around one cross-border region. Do not add it because Portugal feels too small to deserve the whole holiday. It does.