Deutsche Bahn can book reservations from/to/in Spain, either at the ticket office or by phone (030 2970). Other than that, at a Spanish ticket office.

For domestic trains in Spain, you often need to provide the train number (DB need to look up the train via the train number search), which you can find in www.raileurope.com and currently also on the DB website, although not too far in advance.

Reservations for domestic trains in Portugal can only be done at a ticket office in Portugal.

Do you recommend to book the reservations in advance? We will be mainly using the morning or midday trains.

It is usually enough to do it a few days in advance. In Spanish and Portuguese domestic trains, there is no limited quota for pass holders so you can book until the train is full. That does sometimes happen, so it is wise to book at least a day in advance. You could for example book via DB, in advance, your trains for the first day(s) and the rest in Spain.

Seems to depend-judging many reports-and as a little bit of str8 thinking may also conclude: the very long AVE rides from Andalucia to Bcn-often said to be non-available, shrugged off or sold out days advance. There are just 2 or 3/day-one can go nearly ev hour via MAD=change there, pay double for 2 REServ. In main summer ESpana is flooded with passholders on the go.