3-4 weeks in Portugal and Spain

As a general rule: It is much harder than one would expect to travel between Portugal and Spain. Therefore, if you can figure out a way to cross that border only once, you will be saving yourselves some grief. One solution is to fly. I'd use skyscanner to see what flights are available. Your best bets on the Portuguese end are Lisbon and Porto. See what Spanish destinations have non-stop flights. If you have to connect, you'll be chewing up a good bit of extra time and paying more as well.

You need to do some research on what you want to see in Spain. Seven to ten nights is not long, and I'd suggest flying into Bilbao to see it, San Sebastian, and other sights of interest in the Basque Country (you could easily spend 7 nights there). You could then travel west by train (or bus if faster), hitting some of the very attractive cities along the way to Santiago de Compostela. Burgos, Oviedo and Leon are all very interesting. Oviedo is significantly quieter than the other two. Leon is the liveliest. All have very nice historic districts. Galicia itself has a lot of interesting places beyond Santiago de Compostela, which is its premier destination. As you can see, seven to ten days doesn't allow you to cover all that much. Any way you can stretch it to 14?

If you end your Spanish sojourn in Galicia, you will be able to use ground transportation to get to northern Portugal. How to do that comes up pretty often. The deal is that trains are not frequent and are not spread throughout the day; on weekdays, your choice seems to be between ungodly early trains and late afternoon trains. A bus might turn out to be better, or a combination of train and bus. You'll be fine as long as you don't have an extremely tight schedule. Trying to take buses on a Sunday or holiday would also not be a good idea.

One reason I suggest traveling to Galicia after San Sebastian/Bilbao is that the Basque Country is not particularly well connected to the rest of Spain; you can get to Madrid in less than 6 hours, whereas Barcelona takes over 7, so if you feel you want to see one of the two largest Spanish cities, I think Madrid fits better into this trip. However, it's a real shame to get to Madrid and miss Segovia and (especially) Toledo, so Madrid really needs 5 nights, including those side-trips. I assume you can fly from Madrid to Porto.

Including the Algarve may add some complications when you investigate how to get back home. Are you sure it's a must? There are plenty of other good places to see in Portugal that don't take you so far from the major international airport in Lisbon.

I suggest getting an open jaw ticket, flying into either Porto or Lisbon, and out of a Spanish city, such as Madrid or Barcelona. There are several budget airlines that provide affordable one-way tickets between Portugal and Spain. You don't state your interests, so it is hard to give advice on where to visit in each country.

If you spend your first week in northern Portugal, I would suggest you skip the Algarve this time. A week is not enough time to see the Lisbon area plus the Algarve and you would be traveling two of the days of your remaining week in Portugal.
There is more than enough to see in central Portugal and the Lisbon area to fill a week.
As stated by another poster, it’s not that easy to get from Portugal to Spain.

Northern Spain could also fill your entire time in that country. It really just depends on what you want to see. Santiago de Compostela is lovely and the Galicia area is lush and green, and very different than the rest of Spain..
And, since IMO flying between the countries is likely your best option, I would suggest seeing all of Portugal then flying to Spain and back home from there. That would eliminate going back and forth between the countries and wasting that day of travel. Even though the flights are short, by the time you get to the airport early, fly, get from the airport into town, etc., it will take a day out of your time. It’s likely you can fly from Lisbon to Madrid for considerably under $100. If you want to fly to northern Spain you will likely have to transfer in Madrid. You might want to check out the train from Madrid to San Sebastián. That way you could spend a couple of days in Madrid, then train to San Sebastián.

Skip the Algarve. Why focus on the beach when you can to historical things.

We loved Porto and northern Portugal. Also, did a day tour from Porto to Santiago de Compostela. I highly recommend that.

If you can see Salamanca as well just across the border at the end of the Douro Valley.

Spain is a very large country and the best areas are:

1) Around Madrid, include Segovia and Toledo
2) Andalusia, Seville, Cordoba and Granada in the South
3) The Basque Region near the French border north of Madrid
4) Catalonia, Barcelona

All are wonderful choices, but since you plan to visit Portugal, you would likely do best to pick number 1 or 2.

We spent 2 months in Portugal and Spain last February and March, and have returned this year for another month, of March. Last year we did a loop from Lisbon to Faro/Olhao, then Seville, by bus only,
with a daytrip to Cordoba, then Granada, Alicante, Cuenca, Madrid, Salamanca, Merida. Then by bus back to Lisbon, and finally up to Porto. If you look at a map you will see that this is mostly a big circle covering a lot of each country. We highly recommend all of these places. Merida was a big surprise for its Roman ruins. We thought it was just a stopover to break up a bus trip! This year we went to Cadiz for a week and will be going to Lisbon, Tomar, Coimbra and Porto. If you are not in good physical shape, don't spend too much time in Porto. It is straight uphill in all directions. As lovely as Porto is, a nice daytrip on the Duoro is a bonus that we loved. This year we may daytrip to Santiago de Compostela from Porto. Hope this might help.