One ticket for the monorail allows two trips on it. However you are not allowed to ride a complete loop.
For example, you have to board the monkey islands station and depart at Jubilee station. Once you've had a look around that part of the zoo, then you can get back on the monorail and return to monkey islands station.
I don't usually use the monorail because I prefer the walk (and the new bridge has made that easier anyway) however when I visit with younger or older people then it seems decent value if their legs can't walk all the way.
It also offers a different view of the zoo