SIMS covering Pays Vasco & Pays Basque (Spanish & French Basque)

Yes, a single SIM will work for both areas, legislation some time ago required carriers to offer roaming EU wide at no additional charge (Which the plan may exclude the UK and Switzerland, among others).

The number will be a European number of the country in which you purchase the card, but will work fine for calling to or from the other country (France or Spain) Do learn the appropriate Country Codes, since you will need those.

I no longer use SIM cards, so no recent experience with carriers. Both Orange and Vodaphone that you mentioned are popular. You can also look into internet based SIM card providers, like Airalo, and depending on your phone, you may be able to install the SIM as a second line, or as an eSIM

We had one SIM card that worked in both countries during our May 2023 France/Spain trip. The plan we used was Bouygues Telecom My European SIM 30GB, which was a fairly generous data plan for a one-month trip.

To do this before your trip, go to https://www.simoptions.com, then search for Bouygues Telecom, which is a French company. That was a better option for us since most of our trip involved travel through France, with only 8 days in Spain (while in País Vasco). We were careful to limit use of data while in Spain so we wouldn't exceed our 30 GB.

They sent a physical SIM card via US Priority Mail (for additional fee) to our home address a month before our trip, so we were ready to roll once we arrived in France. Things may have changed since then, and e-SIM is becoming more common. Depends how new your phone is, whether you have e-SIM capability, whether you have more than one port for a physical SIM card, etc. We used my husband's phone (which is newer than mine) because it has 2 SIM card ports and an e-SIM for use while at home. It was nice to not need to remove one's "home" SIM card, as would be the case if using an older phone with only one SIM port.

It worked great for us, although about a week after our arrival in France, we got a notification from Bouygues Telecom asking to verify our whereabouts (where we were staying, etc), which seemed odd. How to respond and interact with them was a bit confusing because of the language barrier, but we were able to resolve the matter quickly by asking our hotel concierge in Bayonne for assistance.

BTW, if you don't get a lot of responses to your query, you might try posting this to the "Technology Tips" category, although I have a feeling that you may already have enough info now to help you move forward.

OMG!!! Your posts are a great help!
Re finding one SIM for phone calls in both France & Spain, I saw the following Vodaphone card that looked very good, with the exception of "...100GB unlimited talk in Spain". Do you read this to say the 100 gigs is just a special add-on benefit (vs. the SIM calls being restricted to Spain only)?

https://www.amazon.com/Vodafone-Network-Europe-Sim-Card/dp/B0CFX4PY7N/ref=asc_df_B0CFX4PY7N/?tag=hyprod-20&linkCode=df0&hvadid=692875362841&hvpos=&hvnetw=g&hvrand=7731134167398800941&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9032092&hvtargid=pla-2281435177898&mcid=0f49eecfe9413cc7a81e6cd8fd5c91e5&hvocijid=7731134167398800941-B0CFX4PY7N-&hvexpln=73&gad_source=1&th=1

Re. e-sim: my iPhone allows both an e-sim choice (which I use for my US number/account) and another slot/number for a SIM card for travel. I prefer the card for travel because I don't worry about privacy etc after I leave, as I can physically throw it out!

I saw the following Vodaphone card that looked very good, with the
exception of "...100GB unlimited talk in Spain". Do you read this to
say the 100 gigs is just a special add-on benefit (vs. the SIM calls
being restricted to Spain only)?

Yes, I agree that this is confusing, and you're smart to wonder and think carefully about this, to make sure you understand the limitations, whether there's a "gotcha" lurking in that contract, etc. I seem to recall that we had more generous options for data use while in France compared to Spain (where there would be roaming charges if we exceeded our 30 GB limit). I suspect that's because we chose a plan with a French company (vs. one from Spain).

I wish I could answer your question, but I can't because I don't have enough experience with this. Our May 2023 trip was the first time we ever used a foreign SIM card so we could have data as needed, especially during the week we were driving in France.

You might be able to unearth more info by browsing the "technology tips" section of this forum. (Apologies if you've already done that and didn't find a post that addresses your concern.) I took a look at recent posts, and here is one you might find helpful that discusses a problem someone had with the Orange card and their poor customer service, which might be a reason to avoid that option:

https://community.ricksteves.com/travel-forum/tech-tips/how-to-have-map-navigation-in-europe