"They have a son and a daughter together."
Translation:Sie haben gemeinsam einen Sohn und eine Tochter.
It might either be because of logics (if they toghether have won't necesarely mean that each is the parent to both of them, whereas if they have together is implied they they are their children, both to both parents) Another reason might be due to the form of the sentence, subject on first position.
I'm not a native, but to me, gemeinsam means that some people share something together (like two people sharing a flat or a pack of chips). But zusammen is like a "sum" of something put together - hence the "zu" (so when 10 people go to a cinema together, or when you work together with someone).
I'm sure it can't be wholly applied, there are so many exceptions and weird phrases in German that always throw me off course, but generally, I think this could work for you. (Any natives here? Please correct me if I'm wrong!)
Sie/They are the subject (nominative) who are "having" a direct object (accusative) which could be an apple, a dog, or in this case a son and a daughter. Accusative case is always associated with haben. If you change the idea to holding a boy, it would be: (I have 'him') or (Ich habe 'ihn') as opposed to: (I have 'he') or (Ich habe 'er')
I believe the adverb is required to closely follow the verb, and so in this case gemeinsam comes right after haben. RE your second question: im not sure its possible to provide emphasis by reordering the words following the verb (like, the word order isnt simply an order of descending importance, and i dont know what heterodox ordering would achieve other than confusion); certainly its possible to move parts of the predicate into the first position, and then have the subject follow directly after the verb.
I am also struggling with the word order here, though i don't think the response given answers my question. So: In a different exercise in this set, Duo gave me "Gemeinsam lesen wir die Bücher." to translate to English. Forgive me if that isn't quite right, but the first three words are correct in any case. So why was it acceptable to start that phrase with Gemeinsam, but not this phrase?
I would say either interpretation is possible (the first seems maybe a little better to me), but the sentence is poorly phrased in the first place.
I read the English sentence to mean a couple who together are giving birth to ("having") a son and daughter, but the German "haben" can't mean giving birth. That means it must be one of your two interpretations, but those both sound pretty odd and forced to me, and I doubt the German sounds any better (though I'm not an expert on how the German sounds).
Jane, if you still have this question, I suggest you google nominative, accusative, dative, genitive.
Also, when you click on a bubble to start a new topic in Duolingo, I urge you to click on the light-bulb icon to see a short, very helpful explanation of the material you are about to be tested on. After taking in this information, you can then click Start and be better prepared.