Convocati a Menelao, multi principes Graeciae convenerunt. Parati ad urbem Trojam navigare sunt.
In the second sentence why are the two words making up the perfect passive tense separated by the rest of the sentence?
The second sentence doesn’t have a passive construction. Parati is a plural form of adj. paratus (ready).
Parati ad urbem Trojam navigare sunt.
[They] are ready to navigate to city Troy.
P.S. Yes, you can think that it’s a passive construction. They prepare themselves, so it is a passive participle. Would it be “they prepare a ship”, then you will need to use the active voice.