Is it polite to share an entree or request "doggy bags" at restaurants in Spain and Portugal?

I doubt they would throw you out, or absolutely refuse you, but it would not be common to get a doggy bag. Sharing an entrée would be more common, but not if you were to simply order only one entrée and nothing else.

My wife and I often "Order for the table" meaning we often get wine, bottled water, a starter, then maybe one entrée we wind up sharing, but she may get a salad or soup. Dessert maybe.

One thing to consider is portions are more "right sized", so we find an entrée each is not too much, even with a starter.

Good point. Even though occasionallyh we get huge portions in Europe, it's not nearly as common as it is here at home.

I remember once in France, and twice in Italy, when I indicated Stan and I wanted to split a pizza, we were told "No, a pizza is for one person." So he would order a pizza and I would order a salad, and we both ended up leaving food on the table.

We did have one "regular" restaurant in Padua where the servers knew us, and let us split a pizza for lunch. I doubt they would have done that for dinner.

In Spain, you often can order half portions. Not sure about Portugal.

Covid changed restaurant business worldwide. When indoor dining was prohibited restaurants offered take out meals and bought the necessary packaging supplies in order to survive. Even though most pandemic restrictions are lifted many people are reluctant to sit near others so take out is still common.

The result is that restaurants throughout Europe have take out containers and don’t mind packing your leftovers.

As for sharing, my wife and I, now in our 70s, eat less. We never order two entrees. One orders an entree while the other orders an appetizer or soup or salad to be served at the same time. We each eat half and switch plates. Nobody really cares what you do as long as you are polite.

Sharing is not an issue. I'm currently in Mallorca and last night we shared two starters between four and no-one batted an eyelid. My main course was a perfectly sized portion so there was nothing left over however on previous visits to Spain (and elsewhere) we've sometimes taken away what the kids have left. The restaurant last night even re-corked the bottle of wine for us to take away and finish in the villa.

The restaurant last night even re-corked the bottle of wine for us to take away and finish in the villa

That is the one "doggy bag" we have done in Europe...though only when we maybe over-optimistically, or foolishly, ordered a second bottle.

Both options are generally no problem in Spain. I often share a main with my wife, and that has never been a problem (and I lived in Spain for over a decade).

Remember that portions are not usually supersized to begin with as in the U.S. (You won't see as many obese people....)

In fact, Spain recently passed a law to require offering doggy bags to customers in order to waste less food (though I don't think it has been implemented yet).