You are right that there are two subjects, but they are the subjects of different verbs.
The first clause is
Around 7 A.M. the truck left the zoo and ...
This is a simple structure with one verb having one subject and one object.
The ?? part corresponds to シャンシャンが乗った and is called a relative clause. It describes the noun that follows it in the same way that an adjective can describe a noun. It means "Shanshan rode" and is again a simple clause with one verb and one subject.
I assume you would be happy with 重いトラック (a heavy truck). You could also translate it as "a truck which/that is heavy". In the same way, (シャンシャンが乗った)トラック is "the truck that Shanshan rode".
Around 7 A.M. the truck that Shanshan rode left the zoo and ...
This is an essential part of Japanese grammar. Look up 'relative clause' and learn it well. You will not survive without it.
P.S. I have no idea what Shanshan is. I'm guessing it's a daft panda name or something.