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I finally went there and yes, as stated in JoErNanO's answer, you can visit the museum complex in the morning, go out and then come back in the afternoon to see Corte Vecchia with the same ticket without any problem. In fact, the time slot you select is only relevant for the visit of Castello di San Giorgio (but note this time is strict, so you wouldn't be able to see it if you are late), whereas you are free to visit the rest of the premises of the museum complex at any time within the opening hours, in the same day.
Since the frescoes by Andrea Mantegna at Camera degli Sposi (also known as Camera picta), the most interesting part of the museum, are located at Castello San Giorgio, it's advisable to visit it not too very late in the morning (from my experience, until 10 a.m. is OK) so that there is few people. You have to know that, usually, they allow to stand at Camera degli Sposi only for 5 minutes, but if there is few people doing the visit they allow you to stay there for some more time: in this way, you will be able to admire the frescoes, which are really impressive, in a more detailed way. And you can try to see if you are able to find the self-portrait of Andrea Mantegna among the wall paintings, something which could be a kind of Renaissance version of Where's Wally?.
In fact, from my experience, if you arrive early enough in the morning, you may even buy tickets at ticket office without booking them in advance because there probably would be few visitors.
Upvote:3
After reading and interpreting the information in the official website, my understanding is that yes you are allowed to leave and come back later in the afternoon to see Corte Vecchia. Indeed it opens only in the afternoon so you will not be expected to sit outside of its doors and wait if you are done with your morning visits of the rest of the museum. The tickets are individual and give access to each part separately.
Upon booking you'll be given a time slot for everything but Corte Vecchia so i deduct that you'll be allowed to show up whenever you please for the visit, provided you do so within visiting hours.