Sounds good to me. But Italian actually has words for "both": entrambi and ambedue.
Because the grammatical subject is "viaggiare", which is singular. "Loro due" is the indirect object, not the subject.
I'll bet you that only one in a thousand English speakers could tell you what an indirect object was, and point one out in an English sentence.
Does DuoLingo English teach us how to properly identify parts of speech? Maybe I should be "learning" English, first!
There is a great book called English Grammar for Students of Italian, it's very short and to the point. You can find it here http://www.amazon.com/English-Grammar-Students-Italian-Learning/dp/0934034400