If you want to take the 809 then you must also take the 808 exam. While you may be able to take the 809 before you take the 808 they will not send you the certifications until you complete both.
This path means that you will have to take two exams.
On the other hand the 813 exam is only one exam and at the end you will be at the same place as those who have taken both the 808 and 809 exams.
All that said, make sure that up get a study guide which covers the 813 exam if you are taking that path.
There have been things added to Java 8 such as the Calendar class, default methods, Lambdas, and concurrency improvements which should be covered in 813 study guides.
Or you may be able to get away with getting using an two study guides, and 808 guide and an 809 guide. But this is not recommended.
You should also take a look at the Enthuware question banks which many people have used successfully to prepare for their Java certifications.